Any ideas for a character who is immortal?

Well in Alyson Noel's immortal series the main characters love Damon when he was a little boy i think in the 1400's his father invented an immortal juice and when his parents where killed he completed it and drunk it and so now he's forever immortal he gave it to his lover Ever and now he and she are forever immortal (excluding what happens in the second book) so maybe that's what should happen she drinks it or it's given to her by another immortal some how and it 'killed' her before reincarnated her? I hope i was of some istance to you.