Name: Rainfall(Rainstar)

Rainfall is canon. Here's a tip: After deciding your OC's name, look it up on the Warriors Wiki. If it pops up, it's obviously canon, and you should probably tweak it a little ;3

Age: 28 moons

Young warrior!

Gender: Male

Sexuality: Bisexual

Clan: FalconClan

I haven't seen this Clan name in a while!

Rank: Warrior (later deputy, then leader)

This is a common path for an OC. To avoid being pointed out as a "Mary Sue", perhaps you should stick to later becoming a deputy, and see how the story plays out.

Personality: Calm, patient, very quiet, and strong willed

There are way too many positive traits here. Add more negative personality traits, to balance it out.

Appearance: Blue-grey, muscley, big

I LOVE blue-grey cats! But one question- what's his eye colour?

Backstory: Grew up as a regular cat, was adored by his mother but outshined his brother without wanting it.

Uhm...this is a very, VERY short backstory. I can't rate much of your OC without a longer backstory ;3

Mate: Goldentail

Crush: Used to love his apprentice, Featherpaw, but she had died because of a monster on the way to the Moonstone

Aww :c

Kits: None

Family: Mother (father was leader, but had died while rainfall was deputy) and brother, Nightfall

Uh oh. I sense a biased deputyship. Usually, when a male leader has kits and stays alive when they become warriors, it is frowned upon when you appoint your son or your daughter as a deputy, for it seems biased. You should probably change that.

Other: Not many people thought about him, he wasn't popular among the clan as much as the deputy, or a super dedicated warrior (even though he was one)

Overall Rating: 3/10- OC is a Mary Sue, Needs Improvement

Your backstory, personality, the cause for being deputy, and names all needs development. Change what I suggested above, and you'll be as pretty as a raindrop!