From Sonic Retro
Yellow Zelkova is a character in Sonic X and a member of the Metarex. What he lacks in intelligence he makes up for in overwhelming strength. He sort of becomes Knuckles' rival during the third season of the anime. His name is derived from the Zelkova, which is a type of tree.
Yellow Zelkova is a lot like Knuckles in that he is short tempered and he becomes confused easily. He sometimes forgets his own strength and often breaks things, much to the dislike of his Metarex brethren. He likes to impress his leader Dark Oak and often tries to prove himself to him, much like Storm the Albatross of the Babylon Rogues.
Yellow Zelkova often refers to Knuckles as "hedgehog" which makes him very angry.
In episode 72, Yellow Zelkova makes an attempt to finish off Sonic and the others for good. During the battle, his armor breaks off revealing that he is not a robot and is of the same race as Cosmo, which was unknown to Sonic and the others at the time. When he attempts to get another shot at Knuckles, he misses him and falls into a crater with molting lava in it. Knuckles risks his life to try to save him, but Zelkova gives up and lets himself slide into the lava.
In episode 74, it was revealed that Zelkova was friends with Lucas (Dark Oak) before becoming a part of the Metarex. He is seen in a flashback as part of Hertia's court.