Sihyun has pretty long eye lashes
pretty deliberate considering the author was going for a unique top-bottom/dom-sub dynamic so the artist made their builds as similar as possible.
but it's the eye slant.
top has droopy, downturned eyes (think bts V, chimothee tamales or whatever) where the corners point down and they look permanently come-hither sleepy. often associated with character archetypes that are sweet/subdued/popular/two-faced
bottom has upturned foxy eyes where the corners slightly turn up (usually imitated with eyeliner) and is often used to characterize playful/flirty/sexual/femme fatale archetypes.
Bro at first I kept mixing up seme and uke due to them looking almost alike and holy moly I love power bottoms