There is a five-pointed star in Kokoro's buttons, representing Anger. However, her buttons contain a X-cross (Sadness), instead of a square. Some poster has suggested cards used in ESP tests (such as
Zener cards). As for me, I don't think these geometries represent anything in particular.
BTW, I think of the symbology of Kokoro's buttons is easy to read if you notice how emotions are interpreted in HM:
Anger is a five-pointed star, because it's aggressive. Joy is a triangle, because it's stable, symbolizing contentment with life. Pleasure is a circle, because a life seeking nothing but pleasure is directionless, like a ball that can't stay in one place (going by the description of Koishi's hat). Sadness is a X-cross, because it makes one retreat into oneself rather than opening up.
EDIT:
Another thing, Chabu, the main sprite artist for Mamizou in HM,
said he did Mamizou and Futo's new animations in ULiL. "No matter how you look at it, that thing in Mamizou's screenshot
looks just like the Metal Slug guy, isn't it?"