I won't be around when the new match goes live, so here's some info on Janet:
Janet is a sentient AI-like helper who comes bundled in with every Good Place neighborhood. There is a Janet in every neighborhood in the afterlife; this specific Janet is the one the show follows, who gets repeatedly rebooted and becomes closer to human than any other Janet.
Janet claims to know everything on Earth, from major events to the minutiae of people's lives. (It could be argued that she knows every fictional universe in this contest as a result, but I'm not sure I feel like going down that road.) She has gaps in her knowledge about the afterlife, but it is consistently shown that she is full of knowledge about Earth.
Janet also has the ability to instantly summon any object on Earth, ranging from the mundane to the spectacularly weird. We never see her use this power offensively, though there is nothing theoretically stopping her from doing so.
In addition, Janet can automatically retreat to her void, a formless place that exists outside space and time. She can do this at any time. She can also create other beings who are like her (she creates a "boyfriend" that functions in much the same way), as well as constructs of people who behave like normal people. This isn't perfect, however - the boyfriend in particular is very weird and acts like a defective Janet model. But this is something she can do.
Also, Janet states in the finale that she doesn't experience time the normal way. This is probably a result of the afterlife timeline moving in a non-linear fashion, but also she's been known to retreat to her void and come back seconds later, though a lot of time has passed for her.
Janet is tricky here because she's more of a conceptual fighter. She has a wide array of abilities but she never really uses them in direct combat. The only time we actually see Janet engaged in direct combat is when her powers have been removed due to being on Earth. She's able to easily overpower a group of demons (who look and act like humans) through basic hand-to-hand combat. When her powers briefly return, her attacks are amplified (she's able to easily toss them away). She also, while in the Bad Place, held a demon up by the neck and threw him into a wall so hard the wall cracked.
There are two canon ways to defeat Janet. One is to push on her nose and put a paper clip in a small hole near her left ear; this will turn her into a marble. The other, more interesting way, is that in every neighborhood, Janet has a kill switch located remotely somewhere. If you press it, it doesn't actually kill her - it knocks her out for a while, but then she reboots and grows in terms of sentience. This particular Janet has been rebooted around 800 times, making her far more self-aware than other Janets. I think a kill switch would spawn somewhere in the arena, though whether her opponent would know to target it specifically or not is up in the air. Also, whenever someone approaches the switch, Janet is programmed to plead for her life (even going so far as to show a picture of fake kids she claims to have), though as soon as you walk away, she will admit it's all an act. Here's a video of that:
https://www.youtube.com/watch?v=6vo4Fdf7E0w