Jonathan "Jonny" Cartwright, also known as Mouse, is a member of the Wonderland gang with the innate ability to impersonate anyone, as well as the close friend/brother-figure of Beth Kane and her second-in-command after Dodgson's arrest. He is also the son of Dr. August Cartwright.
Jonathan Cartwright was born in Gotham City to Dr. August Cartwright and an unknown mother. They lived into a cabin located in a rural area just outside the city. At some point in his life, Jonathan had an unspecified accident that left the left side of his face hideously disfigured, causing people, especially other children, to call him a "monster", isolating him and forcing him to stay always alone without any friends.
In January 2003, while he and his father were out fishing, they found Beth Kane, washed up on shore after the car accident that killed her mother. They took her back to their home and when she woke up Beth was immediately terrified, only wanting to see her father again. August tried to calm her down and explained that he wanted Beth to become friends with his son. She constantly screamed at him to allow her to go home, but he only left to get her a sandwich. Then the girl asked Johnny why he covered his face only to learn that he is afraid of scaring people, and she explained that he didn't scare her.
Later, while Beth and Johnny were watching television, he showed her his ability to mimic any voice he hears perfectly, then a news report of the girls disappearance came on and she asked August why he hadn't called the police, however, he never answered but instead grabbed her and threw her into the basement and locked the door, later telling her that the only reason she was there was to become friends with his son.
Later, Beth managed to get to open the door, making her way to the kitchen and calling her father to ask for help, only to be cut off by August who grabbed the phone, ripping it off the cord, warning her that if anyone came for her, he would kill them and throwing her back into her cell right before instructing Johnny on mimic Beth's voice after Jacob and Kate Kane showed up at the house, demanding to know where Beth was. Though reluctantly, the boy executed his father's order and August managed to make Jacob believe that it was Johnny pulling a prank after having heard Beth voice in an video shown in television, prompting Jacob and Kate to eventually left.
Johnny then approached the door of Beth's cell, while she was crying, telling her he wanted to be her friend and sliding the book, Alice in Wonderland, underneath her door in order to keep her company.
Over the following years, Beth and Johnny build a sibling-like relationship, to the point that the boy is said to be the most important person to Beth.
Around 2014, Beth finally escaped from August under unknown circumstances, but remained deeply close to Jonathan.
Sent to Arkham
Working with Beth
Two weeks after his escape, while Beth was held captive by her sister Kate and convinced to tell her what happened to her after she survived the accident, Mouse approached the two while they were stopping at Dusty's Roadside Diner during a trip on the road to the place that marked Beth's destiny and, posing as a waiter, he managed to drug Kate and take her to the Cartwright's old house.
After Jacob Kane and Sophie Moore found out that Kate was held at the shacks, they were confronted by Beth and Jonathan. While Beth took care of her father managing to stab him, Mouse attacked Sophie mimicking Kate's voice and nearly managing to subdue her before she was able to pry his mask off and knocked him loose as he stopped to put the skin graft on his face back on. As Kate arrived, she held a gun to Mouse's head, and entered the house right before Alice could kill Jacob, telling her to decide which is more important to her: killing Jacob or keeping Mouse alive. Alice figured that Kate was bluffing, but Sophie came in aiming her gun at her. Irritated, Alice said that killing Jacob would be too easy and if they all walk out, no one had to die. Kate released Mouse and he leaves with Alice.
Later, Mouse and Alice were reunited once more, and he surprised her with a copy of the same book he gave her many years prior, assuring her that he would escape her pool of tears. Alice said that he saved her, and gave him the face that she assembled from her stint as the Skin Pirate. Mouse said that he loved her, and Alice told her "dear brother" that she loved him too and she can help him became anyone he wanted to be. She vowed that once they're done with Jacob, he would never forget about her again.
Infiltrating Hamilton Dynamics
Sometime later, Alice and Mouse kidnapped Hamilton Dynamics employee Dean Deveraux in order for the first to made a mask of his face using skin grafts, and the second to wear it, mimicking his voice and infiltrating the society's lab posing as him to steal their newest secret project. After Mouse protested asking Alice why they were keeping the real Deveraux alive and she responded they had to keep a low profile due to being under her sister's radar, Mouse claimed Beth was changed and had gotten softer, prompting her to respond that he was the one who was changed, since he used to trust her, to which Mouse responded that he does and would have done whatever she asked him to do.
The following day, he infiltrated Hamilton Dynamics and entered the room in which the weapon was kept, being immediately intercepted by an equipe of scientist and Catherine Hamilton-Kane herself, who complimented "him" for having accomplished such a thing as making their weapon work properly. She asked him how does he resolved a specific engineering problem and, being unable to answer it, he saves himself saying it was a classified information, to which Catherine reacted collegially pointing out that it could not be otherwise being it a weapon designed to penetrate Batwoman suit.
Shocked at the revelation, Mouse called Alice mimicking Kate's voice, thus extorting her the confession that Kate is Batwoman. Angered, he called Beth a traitor due to still wanting to protect her sister despite she abandoned her, and in a fit of rage killed Deveraux using the gun he stole from Hamilton Dynamics. ater that night Mouse went looking at Wayne Tower from the street and Alice joined him to comfort him, explaining she needed Kate for her "tea party" and that he wouldn't need to share her with Kate as they wold share Kate between one another. Heartened, Mouse asked what would happen if Kate "didn't want to play" to which Alice replied that in that case they already have the right weapon.
After the Rifle failed twice in killing Pyra Electrics' employee Derek Holcomb, the last man capable to replicate the weapon he previously stole, Mouse assaulted him on the street and brutally murdered him. Later, posing as Jacob Kane, he infiltrate Crows Security headquarter, where he was approached by Catherine Hamilton-Kane, who told him that the reason why Deveraux and Holcomb had been killed is that they were the only people capable to replicate the coil accelerator, Hamilton Dynamics' secret weapon that can penetrate Batwoman's suit.
"Jacob" agreed to let her know when his men would have found the weapon, and allowed her to oversee the operation after it was found in possession of the Rifle, who has a confrontation with Julia Pennyworth (disguised as Batwoman) after which the latter was captured by the Crows and subsequently freed by Kate. Catherine managed to convince "Jacob" to hold off filing for divorce stating that no matter how difficult everything went they could face it as long as they were together, however, after he returned to the Wonderland gang's hideout, getting rid of his disguise, it was revealed to be all part of Alice's machination to prepare her "Mad Tea Party".
Murder of Catherine Hamilton-Kane
Still disguised as Jacob Kane, while the real one was in Alice's captivity, Mouse accepted Mary Hamilton's request to join her and her mother at Gotham's annual Humanitarian Ball where Catherine would receive the Humanitarian Award. After discovering this, he was confronted by Kate, who asked him if he was getting back together with Catherine and how could he forgiven her for faking Beth's death, however "Jacob" claimed that he's not forgiving her rather starting to understand the reason because she did it; thus making Kate suspicious and prompting her to make him a trick question with a phone call discovering he was an impostor. Right after the girl managed to inform Sophie about her discovery with a message, Mouse drugged both her and her husband Tyler, later replacing all of the Crows agents assigned to the Humanitarian Ball security with Wonderland gang's members.
That night, he went to the Orpheum Theatre with Mary and Catherine, making a toast of champagne in the latter honor while on their limousine, thus poisoning both of them with a toxine developed by Hamilton Dynamics. Later, after Catherine was blackmailed by Alice through her teleprompter and forced to admit all of her crimes in front of the audience before collapsing due to the poison, he transported her to the back of the stage, then, while Mary called the hospital "Jacob" left stating that he was going to look for help.
Mouse then proceeded in keeping out of the building the GCPD while Alice completed her revenge forcing Catherine to sacrifice herself in order to save her daughter's life, giving Mary the only sample of the antidote; then, while an enraged Batwoman confronted Alice, Mouse intervened breaking the fight before fleeing with her.
He displays an obsession with Lewis Carroll's Alice in Wonderland, but is also quite well versed in pop culture; using his ability to mimic voices to cheer up Beth, by references films like The Goonies (1985), by quoting how, "Goonies never say die!"  and then he also uses his mimicry to sound like Emma Watson as Hermione Granger in the scene from the 2001 film Harry Potter and the Philosopher's Stone, when she tells Ron he's saying it wrong and that, "it's Leviosa, not Leviosar…". He is also prone to jealousy to the point where he will kill an innocent person, because Alice is more focused on someone else.
- Voice mimicry: Mouse has the innate ability to mimic the voices of others exactly. He has used this talent to mimic various individuals, such as Beth and Kate Kane, Dean Deveraux, or Jacob Kane.
- Peak of human phyiscal condition: Jonathan is shown to be in top physical condition. He was able to overpower Sophie and trap her in a chokehold. He was also durable enough that he could take a kick to the groin and keep going without slowing down.
- Master of deception/Manipulator: Jonathan is exceptionally talented in the art of deception, as he is able to fool and convince everyone while mimicking other peoples voices or posing as them.
- Expert hand-to-hand combatant/Martial artist: Mouse is proven to be a fearsome hand-to-hand combatant, as he was able to match and nearly best a highly skilled Crows agent such as Sophie Moore in hand-to-hand combat.
- Stealth: Mouse has great skills in stealth. He was able to sneak up on Sophie Moore and right behind Beth without them noticing him.
- Skin grafts: Mouse uses various skin grafts obtained from corpses to create fake faces in order to impersonate other people.
Behind the scenes
- Johnny is the first case of gender-bending in since he's based on the villain Jane Cartwright aka Jane Doe, who first appeared in the Gotham episode "Nothing's Shocking". She was a serial killer and an Arkham Asylum patient, who was under the psychological delusion that her face was hideously disfigured and wore a mask at all times. She had the ability to shapeshift into other people by touching their skin.
- The comics version of Jane Doe is mentioned by Bruce Wayne of Earth-99 as an Arkham inmate.
- She was a composite character and based on two different Batman villains: Jane Doe, a master of disguise with the power of mimicry, and Calendar Girl; a supermodel deluded into thinking she was ugly that wore a similar mask.
- Johnny is also the second case of gender-bending in the Arrowverse where a female character was changed into a man. The first is Lucious Coolidge.