There's no other name that strikes fear into the hearts of men like... Valentine Morgenstern. He says it means morning star, as in "How are thou fallen from heaven, O Lucifer, son of the morning!" (23.29).
We would feel bad about ribbing ol' Valentine for something he can't control, like his name, except for the fact that he wants to wipe out entire races of people simply for being born.
Valentine's Massacre Day
As a young man, Valentine started the Circle of Raziel. Not a sewing circle, but a group of people with similar goals: to exterminate Downworlders, people with half-demon ancestry like werewolves, vampires, faeries, etc. Why? Well, a werewolf kills Valentine's father. But that's like Meryl Streep wanting to kill all mammals because one dingo ate her baby.
In any case, Valentine starts this Circle and has the audacity to include Raziel's name in it. Raziel might as well be God, because in order to create Nephilim, a mortal has to drink from the Mortal Cup filled with Raziel's blood. So what we have here is a man determined to wipe out an entire race of people in God's name. Eesh.
Don't Be My Valentine
Now you know a lot about Valentine. He's someone we hear an awful lot about, but don't actually meet until the book is almost three-quarters over. Once he does appear, boy does everything hit the proverbial fan. Valentine frees Hodge from his curse, steals the Mortal Cup, and kidnaps Jace in the span of about, oh, three pages. He's definitely a force to be reckoned with.
Not only that, but he also reveals that he's Jace's father and Clary's father (which means the brother and sister duo have been canoodling without knowing they were related), but he doesn't seem to have any qualms about killing Clary.
He's a lot more protective of Jace. Maybe it's because Jace is really lost, and Valentine knows he can manipulate him into loyalty by making him feel like he belongs. Also, Jace is a strong fighter and would be a great asset on his side. Valentine wants to dispose of Clary though, because she sees through his little plot, and she has less upper body strength.
Once Clary finally convinces Jace that his father is a conniving creep (to say the least) Valentine does what most evil villains do. He runs away to fight again another day. The coward takes the Cup and escapes to Idris, shattering the Portal behind him. We have a pretty strong feeling (especially when we look at our bookshelf and see the sequel sitting there waiting to be read) that we'll see him again.