*Spoilers for the Dawn Of The Clans series*
This question was asked by ♪M!a♪ for Warriors Wednesday.
Here’s the summary of what he did from Warriors Wiki: (which should answer a some of the questions)
One Eye was a rogue in the forest who was greedy and hungry for power. He falsely befriended Tom, a former kittypet, and the two convinced Clear Sky to let them join his group. However, he was revealed to be controlling and killed Tom when he defended his daughter, Sparrow Fur, from him during a “training session”. One Eye then chased Clear Sky out of his own camp and planned to take over the other groups. When he learned his daughter, Star Flower, was beginning to get close to Thunder, he told her to betray the forest cats by telling him of their plan to defeat him. In the end, the groups banded together to defeat One Eye and sent the rest of his rogues scattering.
One of the things he changed was he made it so that the groups had to work together, so that they learned that they need each other to survive sometimes.
So that does make him a vital role to the story.