Gallifrey ~ Because I love Dr Who, and it's the planet where The Doctor comes from. NN: Galli
Theta ~ Was thinking about the Greek alphabet, and this just stuck.
Sigma ~ See above
Tobias ~ Named after a favourite character in a long running sci fi kidlit series that I still cherish. NN: Toby, Tobitoes, Tibbs ... he's a fish of many nicknames :)
Daisy May ~ She just had this brightness about her and her colours reminded me of a daisy, all springy and pretty.
Kosimo ~ his name means "Cosmos" ... just thought the variant sounded like him, plus he's red.
Rassilon ~ Another Dr Who character :P NN: Razz, Razzle
Luthien ~ After JRR Tolkien's Luthien Tinuviel. I've always loved Tolkien, and she just seems to fit her name so perfectly. NN: Lulu, Lu, Princess