He is brainwashed by Yvone to ruin Penelope. There are few reason why he is an easy target to brainwashed. First he felt guilty about binging Yvone to the festival when she went missing. He lost his grip on her when the firework star and crowd are moving and the're unable to find her (latter it is shown that the Leila take Yvone immediately after she is separated form Derdick so there is no way they can do to find her) and second it is because He FRAKING FALL IN LOVE ROMANTICALLY WITH PENELOPE!!. When all of the main conflict ended Penelope go back to the dukedom to say final goodbye and Derdick who has been stab by Yvone has the balls to confes (or tried I forgot) and told Penelope to stay for him (Yeah right evermore reason to go). In the end Penelope reject him badassly and satisfyingly. He never really in Penelope life let alone her child. If I'm not wrong Reinald is the one that success the family (I read novel MTL so it's not really accurate). He is a stranger to Penelope and her newfound family.
If you feel mad about Renald, don't .....
He was brainwash. Every game character is brainwash by Yvon with those power of ancient leila that's why in the game every male lead love her. Fortunately Renald and the Duke manage to escape form the brainwash and still care for Penelope. Derdick in the other hand completely brainwash by Yvon. In the end of the story only Renald and the Duke that has relationship with Penelope and her child.