Doctor Who's River Song has got a tough time ahead of her — she's joining the biggest science fiction team of them all: the Legion of Formerly Mysterious Characters. Along with Darth Vader, the Man in Black and the Cigarette-Smoking Man, she'll dwell in a place where almost everybody knows all about her.

Almost every mysterious science fiction character eventually loses his or her mystery — unless they get cancelled first. But it doesn't have to be the end. Here are some science fiction and fantasy characters who lost their mystery, but managed to remain cool.

Captain Jack Harkness The leader of Torchwood Three managed to be mysterious twice — when we first meet him on Doctor Who, he's a con-man with a murky past, who can't quite be trusted. And then we discover a bit more about his past as a Time Agent — only to have him die and come back, on Torchwood. There, he's once again a mystery man, whose deathlessness is left somewhat unexplained for quite some time. By now, we know all about Jack's past — although not those gaps in his memory — but he's still ultra-cool.

Walter Bishop The biggest mystery at the heart of Fringe was Walter himself, with his undefined connection to the Pattern and to the interdimensional war that was brewing. Over time, we realized that Walter had actually crossed universes to bring back the duplicate of his dead son Peter, so he could save him. And we've learned a lot of other stuff about Walter's past, including his experiments on Olivia. We even know some of Walter's possible future. But even with his mystery largely eroded, Walter remains one of the coolest, most fascinating characters on television right now.

Castiel When we first met Castiel, he was the first angel the Winchester Brothers had encountered on Supernatural. And Castiel, himself, was pretty mysterious — why had he pulled Dean out of Hell, and what was his role in the Apocalypse? Whose side was he really on? Over time, we learned a lot more about Castiel, and he also made some choices about who he was and what he stood for — but he never stopped being a compelling character — a badass growling angel in a trenchcoat.

The Saint of Killers from Preacher At first, nobody has any clue what this character is about —
at the beginning, it seems weird that a cowboy is running around working for God. It's not until much later that we learn he was a cowboy whose family died, after which he went to hell and was so pissed off hell froze over. Even after we learn the truth about him, he still remains an iconic, awesome character.

Elim Garak from Star Trek: Deep Space Nine He claims to be just a simple tailor, working on Deep Space Nine — but we always suspect him of being somewhat more than that. Over time, we get more and more hints that Garak is actually a super-spy who's in exile for some reason. Eventually we learn that he's a former agent of the Obsidian Order who had a falling out with his mentor, Enabran Tain, and the gaps get filled in over time. But Garak never stops being an ultimate ruthless badass, who pushes his new Federation friends to do what must be done.

River Tam River Song's namesake is the central mystery of Joss Whedon's Firefly and Serenity. What did the Alliance and Blue Sun do to her? What superpowers does she actually have? And why is she so valuable to the Alliance that they're willing to kill anyone who's even seen her? We eventually find out all about River's past, her super-ninja powers and her connection to an atrocity that the Alliance wants to keep secret — but River? Still awesome and fascinating.

Riddick When we first meet him, he's just a mysteroius prisoner with weird eyes. Over time, thanks to the sequel, direct-to-DVD animated movie and video game, we've learned a lot more about Riddick's origins, including his Furyan background and the reason why he was strangled in his crib. We've also learned more about his powers and his relationship to the Necromongers — but Riddick is still one of our favorite ass-kickers.

Caprica Six Unlike certain other characters on Battlestar Galactica, we actually did get a whole accounting of Caprica Six, who started out as a mysterious femme fatale. We learned she was sent to Caprica to get information from Gaius Baltar, and she got the name Caprica Six from the other Cylons, who saw her as a hero — in spite of her conflicted feelings. We got a lot of backstory on this one Cylon character, without her ever becoming less interesting as a result.

Wolverine From his first appearance in The Incredible Hulk #180, Wolverine's precise history was only vaguely alluded to: initially only revealed to be an agent of the Canadian government, he then became known as the guy birddogging Jean Grey from Cyclops – and as his popularity grew, so did his backstory: Weapon X, "James Howlett", Sabretooth, etc. – He even survived Wolverine: Origins! The character still retains much of his mystique today: in the latest story arc of his own solo series, Wolverine unknowingly slaughtered his four illegitimate children.

Mr. Freeze Making his debut in Batman #121 as Mr. Zero, Freeze began as an also ran ice-themed villain of the DC universe, dressed in an oddly mismatched lime green and bright pink outfit with a Kermit the Frog neck fringe. Though the character was always dependent on sub-zero temperatures to survive, his transforming incident was originally described as self-inflicted — an accidental side effect of developing his ice gun. It wasn't until the producer's of Batman '66 decided to use the character (after deeming Two-Face too gruesome) that he was retooled as a tragic victim – in this case, one that Batman felt responsible for creating, dousing Freeze in the cryogenic chemicals during an arrest. While the series boosted the character's popularity (he was tied with Egghead for most episode appearances behind Joker, Penguin, Catwoman, Riddler and King Tut, after all – one more than Egghead if you discount "The Enchanting Dr. Cassandra") it was Batman: The Animated Series' take on the character that skyrocketed him to one of Batman's most popular enemies, revealing his criminal identity was a response to the murder/later not murder of his comatose wife. Just five years later, Mr. Freeze was the headlining villain of Batman & Robin. Freeze's origins weren't exactly revealed as retooled (though every alteration became canon immediately), but he's a terrific example of expounding an origin instead of belaboring it.

The Doctor By the end of the first Doctor Who story, "An Unearthly Child," we're left thinking the Doctor could be an insane, kidnapping pimp with an underage girlfriend, and it just gets more insane from there. We only find out he's a Time Lord much later – as well as why he left Gallifrey, and why he does what he does. Ever since "The War Games," there have been attempts to add more mystery back to the character, including the show's latest episode. But it almost doesn't matter — even if we already know the Doctor's origin, he's still one of the most interesting characters in science fiction.

Al Calavicci We kept discovering more and more about Al throughout the entire run of Quantum Leap, right up until the final episode – he was an astronaut, boxer, pilot in Vietnam, circus performer, POW, the subject of a Pulitzer-winning photograph, married five times and was even acquitted of murder. And everybody loves him.

Agent Dale Cooper Though the second season of Twin Peaks was unfavorably received at the time, the series has gained second life over the years, and Cooper's history with his psychopath ex-partner Windom Earle and romance with Earle's deceased wife Caroline dominated the back half of the series. And Dale Cooper remains a beloved character.

Angel When we first meet him on Buffy the Vampire Slayer, he's a shadowy figure who knows more than he's saying, and he seems to be there for Buffy whenever she needs him — but who is he? We eventually learn he's a vampire, but it takes a couple seasons to get the whole story about how he got his soul and why he's obsessed with Buffy. But even after countless flashbacks on BtVS and his own series, Angel still has a certain ineluctible cool factor. (Despite the ill-advised attempt to give him mystery again, by turning him into the supervillain Twilight.)

Rorschach in Watchmen He's a weird vigilante with a crazy mask who seems to come out of nowhere, investigating the murder of his friend the Comedian. It's only over the course of the story that we learn the full history of Walter Joseph Kovacs, including his abusive childhood and his early violent outbreaks as well as the reasons he eventually becomes a vigilante with a mask made of shape-shifting fabric. But we never stop being fascinated by Rorschach and wondering what he's going to do next.

Thanks to Annalee, Cyriaque and Kaila for the suggestions.