Is this character allowed?

[Madotsuki]

I'm not sure if this character is too similar to the actual Chara here, but if I ever play a different character, I'd like to try Boss Chara from the Boss Chara AU. Would this be like a "Link and Dark Link" scenario, or are they too similar?

[Kyle Crane]

Actually, something similar was brought up, and with the same character, too. Someone wanted to play as Underswap Chara as their alt. It was ultimately decided that, no, it wasn't an option - however, they COULD, say, choose Swap!Asriel and have Swap!Chara as a secondary.

[Madotsuki]

Aw... well, I can always use her in the Secondary Saga, I guess?

[Alex]

If you're a character from Undertale, you most certainly can.