ok, thanks for the input!If you want personality then a Blue Ram is the way to go, but a 10 gal would be too small I think and they prefer pairs. I would suggest another betta but otherwise maybe a dwarf/honey gourami????... though not sure if that would work so I'll leave that for someone with more experience.
This would be a separate tank for my sister who fell in love with my fish as well. She's not really looking for something quite as big as a 20 gallon. I've looked into pea puffers, and they'd be perfect but I'm afraid of it getting bored as they are very active and intelligent. I've seen them bite at any other plants or invertebrate just for the heck of it, which wouldn't be a massive issue if it weren't for their sharp teeth.Are you setting up a second tank or would this be a tankmate(s) for your female betta?
Do you already have tank or planning on purchasing? If you haven’t got tank just yet you could also look into stocking options for 15/20 gallon tanks. That would give you some more options for fish, especially those that are best in groups.
Have seen info about doing dwarf pea puffers solo in 10 gallons. The bettas are hard to beat in terms of sass and personality. I named my female Prima Donna and it really suited her.