Danny / Mondo

[Fusion of Mondo Gecko and Danny from the first 90s live action film]

Danny is Mikey's.. rival? Frenemy?? She steals his phone and runs up a wall and he has to desperately try to lasoo her down while she tries to guess his passcode. She's a 13 year old runaway at the start of the series who's with the Foot Clan

If you're wondering "Wow how are you going to fit the extremely depressing topic of being a runaway child into Rottmnt's lighthearted comedic tone?" the answer is I am simply not going to address it because there's no way to make that funny. The way she starts hanging with the Foot is essentially by begging them into letting her do odd jobs (ie stupid errands) for them. She's not actually a full member of the Foot until her 13th birthday when they let her join officially though (The Foot in Rise seem like they'd have those kind of rules lol). And when she's actually a full member she's still doing stupid shit but at least she's got a home now

Danny's got an innate talent for pickpocketing, which is partially how she was getting by before she joined the Foot, and once she joins them for real they sometimes ask her to do stuff involving that.

She's the youngest member of the Foot by a few years, so depending on the person she's interacting with they'll usually either A) treat her like an annoying pest or B) kind of dismiss her because she's just a kid (which she is but she hates being reminded of it). She doesn't really connect with her peers much until she asks to join some of them who are heading out to skate once they're off-duty and their response is "Uh. Sure Danny. If you want."
Turns out she Loves skateboarding so she starts connecting with this little group of Foot Clan recruits who sneak out to skateboard a couple of times a week.
They're all older than her still but finally she has friends and it's very nice and really cements her loyalty to the Foot- so when the turtles roll around she's ready to Throw Down. Her ability to Throw Down is limited. She particularly focuses on Mikey as they're both the same age and she finds it fun to annoy him.

She gets mutated pretty quickly after meeting the turtles- if this was in the format of the show there'd be a couple of episodes with un-mutated Danny and then the oozesquito gets her
The reason she turns into a gecko is because one of the people in her skate group has a pet gecko and it was the last animal she interacted with before she got injected

At first Danny is terrified to go back to her new friends as she believes they'll reject her due to her mutation. Despite everything Mikey is actually the one to console her, and tell her that her friends care about her for what's on the inside, not the outside. She's reunited with her friends who don't care at all and it's very heartwarming and then she goes right back to being a pest to Mikey. So the Foot just has a gecko member now.

For a while things are good.

Over time though (and as the seriousness of the Foot Clan's plans begins to ramp up) Danny starts to get cold feet about what they're doing and after the events of the season 2 finale she leaves what remains of the Foot, meaning she avoids being Krangified during the movie, but she is once again homeless and friendless. 

During the hypothetical season 3 she starts to have an arc where she grows closer to Mikey. Her first appearance in the season is while the turtles are helping to round up the stragglers of the Krangified Foot Clan that managed to escape the EPF’s clean-up operation. They watch Danny from a distance as she tries desperately to communicate with the Foot members, and it is revealed that they are what remain of her old friends. They behave aggressively towards her however, and the turtles plus April are forced to intervene. After capturing the Krang zombies, they talk to Danny, who is Going Through It Badly and isn’t herself at all. Mikey persuades the others to let him bring her back with them (and then they have a whole arc where they gradually become besties but it’s difficult for both of them because they both have their hangups and Mikey is using Danny to distract from his own issues and Danny relapses back into thieving but they work it out etc etc). 

At some point while staying with the turtles she meets Warren Stone, and it turns out he used to work with her father before he got mutated (Warren bluntly tells her he didn’t like him).

Danny eventually moves from the human world to the Hidden City. She starts helping out at a skateboarding club for yokai kids at weekends, which she ends up taking over after a few years when the old coach retires. She starts to struggle to keep the club going (which has come to mean a lot to her) and to keep a roof over her head, which is when she falls under the thumb of Big Mama, who offers to finance the club in return for a few favours. This quickly escalates as Big Mama asks for more dangerous and overtly criminal favours and blackmails Danny into keeping quiet by threatening to turn her over to the Hidden City Police and close the skate club. 

Mikey finds out about what’s been happening when he tries to stop a break-in and unmasks Danny as she’s beginning to escape. He’s pretty pissed about the whole situation.