Karkat (full proper name Karkat Vantas) was the first roleplayer on Ponychan, and opened the doors to numerous other roleplayers who eventually moved to the /rp/ board. Karkat is a character from Homestuck, and helped to make the web comic popular amongst other bronies. Roleplaying as if she was a normal poster who was actually Karkat, this was a topic of controversy for some time. To give the illusion of being that character, she would often type in all caps, something that other Homestuck roleplayers continued, but with other typing patterns.
As roleplay became more popular, she took on one or several alternate identities. Both she and her brother posted on the site during its first several months, but it is possible they may have left. For full information on the actual character, go here.
Note that there are now multiple Karkat vantas in /rp/, it is not known whether if the first Karkat is still in/rp/.