Skins/WMG

Grace
I wouldn't put it past David Blood to lie to Rich just to get him away from her...

Matty is a hardcore stoner.
It explains his Dull Surprise and monotone, and his actions in the pilot of season 5- he comes across Franky and, in his drug-addled haze, hangs around despite the fact that she's distraught and wielding a gun. It also explained why he looked surprised when he said she was beautiful- he'd actually had a coherent thought.

Cassie is an alternate universe version of River Tam
Or a distant ancestor or earlier incarnation of her. I mean, seriously, they are very similar characters - both very intelligent to the level of being scary-smart (tell me the thing that Cassie did with her A-level exam of seeing how long she could wait to start "and still get an A" isn't something River would totally do) but very mentally-disturbed, and also Cloudcuckoolanders, and Cassie's ability to read people and manipulate them borders on the psychic (see: the scene where she tells Jal she knows she's  - as far as we know, Jal hadn't told anybody by that point). Also, they share the same basic body type, even if they have different hair colors, faces, etc.

Effy is a Time Lord
Because somebody has to be. But what would her TARDIS be? That stuffed giraffe?

The characters who die each generation are all Time Lords and when they "die" that's actually them leaving and just creating a new incarnation of them
This would also explain why  made his first appearance the episode right after, he really is replacing her in the gang as the resident Time Lord.

S5!Franky and S6!Franky are actually different people
Like maybe Franky secretly has a twin and they've been switching off with the cast.

Franky is a Stepford Smiler who is trying different masks to hide her issues
Like Effy was. This would explain her personality change in S6.

Grace is a ghost in S6.
That is why everyone halucinates about seeing her. They aren't all mad she is communicating with her friends.