le
/le/ · A1 · pron
to him, for him
le = Latin illi (to that one): the L of él and ella plus a dative e
From Latin illi, the dative of ille (that one). Spanish kept only the tail: il-LI became le. One form serves him, her and it, because Latin illi never marked gender either. The to/for sense is built in, so no extra a is needed.