Page 234 of the MIC has a listing of 'common item effects' that can be added onto existing magic items at no extra cost. In my case, the Vestment of Many Styles has a +1 Deflection bonus to AC and a +1 Enhancement bonus to Natural Armor added on, and the Healing Belt's +2 Enhancement bonus to Strength is also one of the available options. It's a good way to save money and get the most out of your equipment slots at higher levels, and is pretty fun to make use of at lower levels, as well.
As for thoughts on a shared background... Andiliat could have been taken from the Material Plane as a child, perhaps, raised by the Fey while a Changeling child was left behind in his place. We could say that he was raised among Seelie Court Fey, that when he grew up, he became a sort of Fey Knight, and champion duelist...
Then, while acting as a second for a senior duelist one day, perhaps a 'handsome nobleman' of the opposite court caught his eye, and since he was Chaotic Good and highly impulsive back then, he stepped into the fight and killed the duelist that had been set up opposite him. Offering to 'make amends' for this rash act, he entered the Unseelie lord's service in replacement of the knight he had killed, with the intention of wooing him.
Of course, I'm sure you can imagine how well this ended up going down... And I'll leave it up to you, whether he was first turned to his current Lawful Evil alignment by Eiseth's exquisite torments and then infected with entomanothropy, or whether it was the magical disease that ultimately changed his outlook on life. ^^