The easiest character to forge a bond with would be Skelly. Though ThePuffyPenguin is correct in that doing this with one of the characters in the House of Hades is a more consistent strategy than doing this with one of the characters you meet during runs, most of them will be missing from the House every now and again; the only character that doesn’t do this at random is
Skelly is the exception, he will always show up in the room where you start your runs. To forge a bond with him, you will need
6 Nectar and 3 Ambrosia. After you’ve given him 6 Nectar, you will need to fulfil a favour in order to start giving him Ambrosia. The conditions regarding that are below (and censored for those that want to simply stumble into it).
*** Spoiler - click to reveal ***To begin, you must unlock 4 Weapon Aspects, which will only be available to you the run after you unlock your last Infernal Arm. After a run or two after you’ve done this, Skelly will request you to kill him permanently. To do that, you will need to max out the default Aspect of the Stygian Blade, which will cost you 4 Titan Blood to do, 1 per level. If you’re one weapon clear away from completing the Master of Arms prophecy, go for that, as the prophecy will grant you with 10 Titan Blood upon completion. Once the default Weapon Aspect of the Stygian Blade is maxed out, simply kill Skelly with the Blade.