I haven't read the books, but I played all games multiple times. What made me like her?
-She is definitely head over heels in love with Geralt, so much that she is ready to betray everything and even her best friend. Although I haven't read the books, from the sections people posted it is clear that in them her love was one sided, I think Geralt cared about her, just not in a way she wanted. And as some people posted here at the end of the books she is left in a pretty miserable state, having betrayed her friends and with the love of her life dead and gone. Enter games, when she found Geralt she must have been ecstatic and with his memory gone and her not knowing what has happened to Yen she saw an opportunity she couldn't miss. I know that for many people this is seen as a shady move, and it is, I can understand her and it shows me that she is ready to do anything to be with Geralt, even though her love can be easily seen as obsessive.
In TW2 I got a vibe of insecurity from her regarding the relationship with Geralt, knowing his memory is returning and that Yen is alive, she is aware that losing Geralt and even having him resent her for the things she has done is a real possibility, especially after Geralt demanded her to come clean and tell him everything. Despite all that she agrees to help him, but as we latter find out that the Rose of Remembrance also has mind control abilities I can't help but wonder whether she was honest in her desire to help Geralt regain his memory or is she maybe thinking about doing all she can to keep him beside her. While the game never implies that openly, it is still something I took in consideration. She continues to seem insecure when she asks Geralt whether he wants her to help him find Yen and even if the player continues to reassure her of the love he feels for her she still, at least to me feels insecure. What led to their breakup between games can only be speculated but with Geralt memory back I like to imagine they mutually decided to give each other some time and see what will happen with their feelings once Geralt is reunited with Yen.
In TW3 that insecurity is still present and can be seen through the awkwardness that exists among them when they finally meet for the first time after TW2. It is clear that her feelings are very strong but she fears Geralt doesn't share them and is reluctant to react. Transition between that and the rekindling of their love is very well done in TW3 and that I find very appealing. So bottom line, she is ready to do anything for Geralt and while her love is sincere, things she would to for it are not, and for whatever reason my demented brain loves that
-The facial animations in TW3 are among the best I have ever seen in a game and even a look or a smile can tell a lot, you can almost imagine what that person is thinking and Geralt and Triss share many of those in the game( after the kiss at the ball is a great example and again latter in the stables).
-She looks beautiful, her face is TW3 really made me fell for her.
-She also has a very sweet personality and I find her funny and lovable.
I usually avoid writing long posts like this because English is not my first language and it is hard to express some things in detail like I would be able to do on my native language so if something sounds off sorry.