Previous travellers

Ryan:
Ryan was originally called into the multiverse by Sally, who was desperately calling out for aid. This is a classic example of 'be careful what you wish for'. He initially travelled with Sally and Ginnie, and later MattMatt and Elle, but his Embrace at the hands of a Cappadocian Methusulah, his vampiric nature, his frequent killings and his acquiring a demonic master from the Dark Between the Stars drove an increasingly large rift between him and his fellow travellers. Eventually only Sally (who was going insane at a rate of knots) could work with him, and sadly did so for much longer than she should have done, only leaving his side when he and Ginnie came into conflict and ended up trying to kill each other. Ryan is still out there, somewhere, serving the Dark Whisperer In The Void, controlling his race of powerful (but stupid) vampiric childer, and possibly acting as the avatar of Baal and Baali antediluvian in at least one of the 'verses.

Elle:
Elle entered the multiverse half way through Season One, and was a regular character until the end of that season. Elle's powers were primarily fae in nature, although no one (least of all Elle) understood them very well. She seemed to be able to affect the emotions of those around her (once acting as Eros in a nightclub), and also could create and control chimerical beings of some kind. She additionally had some kind of runic magics, which could be used to open gateways through which the party could travel.

Elle died, along with Sally, MattMatt and Ginnie, on Severus when a gateway she was opening malfunctioned and threw them all into the void of space. Unlike the others she was not brought back to life by the Dark Whisperer In The Void/The Angry Space God and is presumed to remain within its realm.

Christa:
Christa was a briefly appearing character in Season Two, who was involved with Ryan and had some form of reiki based magical powers.

Andy:
Andy appeared shortly after the start of Season Two. He was a forceful personality, used to being in control of his situation, and found it hard to come to terms with his position as one of the neophytes of the world hopping group. By Season Three, this was bringing him into regular conflict with both Sally and Ginnie.

Andy initially seemed to have some kind of technopathic powers, but soon after the nature of the multiverse was explained to him he came to believe that it was possible to will yourself into some kind of different powerset. He also believed that it was possible to control and manipulate the multiverse's natural tendancy to force assmilation on those who travel between worlds. He did have some level of success with this theory, assimilating into an Exalt whilst staying in a WoD 'verse, and later seeming to develop Exalt-based powers, including shapeshifting into a giant half man-half orca.

Andy died on Arran, fighting Ryan's spiders. It is unknown whether he was brought back to life by the Karma Police. He may be dead, or he may simply have chosen to not make amends for past deeds and is currently living quietly in the worlds.

Tim:
Tim appeared partway into Season Two, and proved a somewhat chaotic addition to the group. Most noticeably, he nearly destroyed the entire multiverse by changing a poppy in the poppy field at the centre of the multiverse.

Tim has never died in the multiverse and his present whereabouts are unknown.

James:
James was brought into the multiverse by Ginnie to provide Sally with some kind of emotional support after she went mad at the end of Season Two. He settled with Sally, Ginnie, Andy and MattMatt and re-entered the multiverse with them after they got their minds back.

James was a forceful personality who often came into conflict with Sally and Ginnie, especially over their use of the phrase 'Senior Partners' to describe those who had been travelling the multiverse for longer. He disliked their assumption that they automatically knew more than him and this regularly lead to trouble.

James had a powerset which seemed to be primarily based on the Deadlands 'verse - he could summon spirits, manipulate luck and do tricks with cards. He also had hyper-enhanced vision, and some form of beast mastery, which may have been linked to the Earthdawn 'verse. He was also reforged into a blood elf by Sally after she saved his life using lifeforce from the bloodwood she had earlier unintentionally summoned.

James died on Arran, fighting Ryan's spiders. It is unknown whether he was brought back to life by the Karma Police. He may be dead, or he may simply have chosen to not make amends for past deeds and is currently living quietly in the worlds.

Ben:
Ben made a guest appearance in Season One, when he was briefly summoned into the multiverse by Matt. He was noticeably in much better physical shape there that he was normally. However, the power by which Matt summoned him out of his own world was only temporary in nature and so he was able to return to his own world.

Nick and Mandy:
Nick and Mandy were friends of jez from his Keele days, and had some experience with the theory, at least, of the multiverse before being drawn in. They were drawn in from their lives in Australia, but only stayed for a while, before Nick successfully willed them back to normality.