Sans:

He'd be cute at first, but soon you'd be annoyed at just how lazy he is and it would spark an argument

Frisk:

They're a child

Chara:

They're also a child

Asriel:

They are also a child

Papyrus:

He wouldn't understand and treat you like a best friend, eh plus he likes Mettaton

Mettaton:

He has way too many fans, so he would probably brush you off as one of them

Toriel:

That wouldn't really work out. She's probably a lot older than you

Asgore:

He probably is still trying to get over Tori and his dead children

Undyne:

She likes Alphys

Alphys:

She likes Undyne

Muffet:

She's probably be good, thought I think she would make you pay her to date, idk...

Grillby:

Meh