Jake Simmonds
Jake was Rickey Smith's friend (and in a deleted scene, possibly a lover, implying that he's not entirely heterosexual) until he was killed by Cybermen, which upset him deeply. When he and Mickey Smith became friends, Mickey made it clear that he wasn't replacing Rickey in any shape or fashion, but expressed sympathy with Jake's loss.
Eventually the two became friends and co-workers at Pete's Torchwood, forming a close brotherly bond. When Pete's world died and Jake was taken to the main universe, he didn't have a place in Cardiff and asked Alex if he would mind if Jake joined him in the 23rd century.
Alex agreed and Jake left for the 23rd century, working from there instead.
Eventually the two became friends and co-workers at Pete's Torchwood, forming a close brotherly bond. When Pete's world died and Jake was taken to the main universe, he didn't have a place in Cardiff and asked Alex if he would mind if Jake joined him in the 23rd century.
Alex agreed and Jake left for the 23rd century, working from there instead.