Personality-wise, I'd go with Faye. If there's one character type I absolutely hate, it's the satellite character, and Faye has to be one of the worst uses of the trope I've seen in modern media, let alone the Fire Emblem series. To be fair, I tried to like her at first, but as I began her supports with Alm and Silque, her obsession with the former became so. Goddamn. Irritating! I know there are other characters in the series that may also qualify as "satellites", but the difference there is that they have other character traits to fall back on.
Gameplay-wise, I'm going with Python. He has consistently proven to be the most useless character in Alm's story. Python not only sucks at dealing damage, but in almost every battle I've used him in, he dies. As a result, I end up wasting turns on Mila's timewheel because perfectionistic nature forbid I complete the battle with one less unit. Honestly, I'm beginning to wonder whether I should start leaving him behind in every future map.