I would go for Isabelle. I know that she's not getting good feedback lately because of her limited role in New Horizons. However, she's still adorable and a keeper!
Gosh it's so hard to pick just one, but for personal enjoyment and attachment/nostalgia, I think it'd be cute for Tortimer to retire and chill out on the island full-time. Maybe just hang out on his porch all day. Invite Frillard over for old man shenanigans. Bond with the crankies. Tell us stories from past games.
I think Katrina's house would look amazing and she's a close second because I've always loved her.
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Edit: oh who am I kidding my heart will always belong to the Czar of Halloween. I am so excited for October and a Jack update!
celeste & rover. celeste is the most adorable NPC. i SAID what i said!
i would have said flick, his design is so beautiful, but i wanna keep selling bugs to him and getting models from him. and i do prefer him more than nat.
yeah leif is also adorable, i wish we had more sloths. a baby.