Claude was my fraidy fish and would not flare at anything (his reflection scared him), until I tried a blue pencil.
Aryen will only flare at M.A.C., but not much. I think he's still young.
M.A.C., another fraidy fish, didn't flare at anything until I came to visit my family (my sister's taking care of him). He's been flaring non-stop at me since!
Corona and Joachim (recently passed away) flare at EVERYTHING! Joachim would always greet me by flaring.
Renard flared at Pony Boy and Corona when i first got them, but no one else. He was in a tank where he could see my sorority, but completely ignored them. He'll flare at anything shiny I put in front of his tank. :P
My new copper boy flares at Renard, but not Corona. He flares at his reflection too.
And Roald Dahl, my sweetest betta, did not flare at anything at all or give any hint of aggression...until I added my new HM boy. He flared at him! Weird.
I haven't tried my new HM yet, but he hasn't flkared at Roald Dahl or Corona.