I'm 16 and when I was 6 (around 2006-2007), I used to watch a cool anime (don't ask me what channel because i found out it was illegally downloaded by my other friends).
I only vividly remember certain aspects of what it was like. I think there was a boy with maybe some kind of gem that I think he wore on his forehead, and he could transform into a grown man (again, I think so). Then there was a fat cat who also had a gem; he could transform into a tiger. I think there was a third person where, all together, they combine and make one powerful grown man. It's been bugging me since I was 11.