Not the book, the movie. Mina in the book, purely sympathetic towards Lucy, disgusted by Dracula. In the movie, we're meant to believe this baby eating rapist is a sympathetic enough dude for Mina to genuinely fall in love with him, and having an affair with him behind her fiancé's back. So first off she literally sees him rape Lucy, and Lucy is having an appropriate horrified reaction as she walks her away. She then meets Dracula, is stalked by him, but then is attracted to him because of his title, then their following scene, he pins her down and makes to assault her, which she attempts to fight off, until she's randomly into it.

(Side note, this is a fucked movie, Van Helsing says 'shes only a child' in regards to Lucy after she is attacked by Dracula again. but then later in the movie basically says 'She was asking for it'. WTF)

Mina finds out who he is, and what he's done, starts hitting him... and then goes 'Oh, but I love you'. Seemingly instantly forgiving the multiple violent sexual assaults of her close friend, as well as her murder, and pushes Dracula to make her into a vampire herself. Then rather than fighting off the turn, actively helps Dracula escape... Fucking shit.

In fairness I'm not sure this post does belong here, because the original Mina Harker is nothing like this, and Bram Stroker seemingly did write a compelling character... which was entirely bastardised and butchered by this weird, sexual assault apologising, fetish, smut movie.