You suppose rightly, Light and Wood are Serin's elements!
If you have Wind coming up however, it's a hint you have enough affection points to get on Nazir's route. If you want to succeed in Serin's route, your Light has to be on 30 and Wood on 40.
(We are considering lowering the stats since if it might seem to high for the first check >w< )
Edit: Also, there's been a bug reported on Week 11 which might make it impossible for you to succeed Serin's route! If you update the latest Demo and start again before Week 11 (any savegame from before that should be fine), it should be ok now! :D