There are a lot of characters in original Trek who appear once never to be heard of again. Do you have any that have made an impression and stuck with you for longer though? What makes them your favourites? 
Doctor Miranda Jones from Is There In Truth No Beauty is definitely up there for me! I think her story and ambitions are interesting, and overall I think the episode handles her blindness with tact. Diana Muldaur plays her wonderfully and I was happy to see her back on board as Dr. Pulaski in TNG (sad to see her go as well!)
Another character I like and wish was brought up again is Alexander from Plato's Stepchildren. He's a very sympathetic and brave character who I hope found a place where he's respected.

Doctor Miranda Jones from Is There In Truth No Beauty is definitely up there for me! I think her story and ambitions are interesting, and overall I think the episode handles her blindness with tact. Diana Muldaur plays her wonderfully and I was happy to see her back on board as Dr. Pulaski in TNG (sad to see her go as well!)
Another character I like and wish was brought up again is Alexander from Plato's Stepchildren. He's a very sympathetic and brave character who I hope found a place where he's respected.