Fuzznip informed me earlier that the grammar standards article on-site dictates we should be using 'who' for Pokémon, i.e. we refer to them as a person. Pokémon are essentially animals (pocket monsters etc.), and unless we're speaking anthropomorphically, we should stick to 'which', which many people currently use. This is also consistent with our policy of using 'it', unless the Pokémon has only one gender (Blissey is a 'she').