So PASSION has a total of 3 novels that focus on Taeui + Ilay:
PASSION: Taeui +Ilay's initial meeting and beginning of their relationship
Diaphonic Symphonia (commonly referred to as DS because who's gonna write allat lmao): Taeui + Ilay's relationship after 3-5(I think?) years, with a new story and 2 other characters Richard(actual scumbag) and Chris. Tbh I didn't like the 2nd couple at all, and neither did 90% of the fandom purely because of Richard. Mainly read for the Ilay and Taeui moments
Suite: the last part of the series and alot of focus on Taeui + Ilay. Has the best smut and romance for the two ლ(´ڡ`ლ)
Shows how much their relationship has grown and takes place (5-10? Iirc) years later

*spoilers for chp 140+ below
When I read the novel, i was really hoping that Taeui would get a chance to explain himself, but he didn't and end ended up getting raped for the 4th time
I really hated illay at this time
I really wish the author didn't make their reunion be this way
Their relationship later on in DS and Suite (my favourite) was so well done, it's a shame PASSION wasn't the same...you'd think by the end, the 'humanity' that Ilay earned would let him listen to taeui instead of jumping to conclusions