I was going to go for Oz again although he actually did get a fair bit of screen time, he just sadly wasn't well enough used (see also Dawn) - there were plenty of one-offs I would happily have seen more of (Hazel mentions a couple)... but I'm going to pick one who was I think only in two episodes and they were both pretty good (the second one was one of the very best) so... the Character Who Didn’t Get Enough Screen Time is Vamp Willow