KOF 94 - 97 is known as the Orochi Saga.
The main character from '94 to '97 is Kyo though there are other characters introduced in the later games who are vital to the storyline.
'95 introduced Iori, a member of the Yagami clan who is the nemesis of Kyo's Kusanagi clan.
'96 introduced Leona and revealed that she is a member ot he Orochi bloodline. For the final battle of '96 Kyo and Iori teamed up. After defeating Goenitzl Iori entered the Riot Of Blood (this is what happens when the power of the Orochi blood awakens) and disappeared.
'97 introduced Shingo Yabuki, a former student of Kyo and Iori. Also new to the tournament was the "New Faces" team (Yashiro Nanakase, Shermie and Chris). At the end of the tournament the New Faces team revealed they were of Orochi Blood as well and that they were 3 of the 4 Heavenly Kings Of Orochi (Goenitz the boss of '96 was the 4th). At the end Yashiro and Shermie killed themselves, Chris gathered their enegery and transformed into Orochi. Orochi battled Kyo and threw him to the ground when Iori appeared and once again teamed up with Kyo. They fought Orochi but he used his power to once again awaken the Riot Of Blood in Iori in order to turn him against Kyo. But Iori turned against Orochi and Kyo made one final blow to defeat the distracted Orochi.
'98 is a dreammatch. It's not part of the KOF storyline. So you could choose one of the fighters you find the best artwork for to be on the cover. I'd like to see Shermie, Yamazaki or Heidern but I don't know how easy it is to find decent artwork. You could also go for a group shot or maybe no fighter at all on the cover. It's your call.
'99 starts the NESTS storyline. The most notable new entry is K', he is the main character of '99. The game's Hero Team was K', Maxima (another new entry in '99), Shingo and Benimaru.
I know that this was a rather long answer to your simple question, but for a fighting game KOF has a pretty estensive and elaborated storyline
