The everyday reading is kǎi. 楷书 is the clear, standard 'regular script' learners practice first; 楷模 means a role model. The tree reading jiē is separate and rare.
Left wood radical, the indexing component. The character began as a tree name; that botanical origin is why a 'tree' radical sits on a word now meaning 'model' and 'standard script'. Same family as 林 and 树.
Right 皆 supplies the sound — jiē, kept directly in the jiē reading and shifting to kǎi in this sense. It contributes no meaning here; the same phonetic appears in 谐.
