So, before we begin, I need to inform you all: I have not actually watched RWBY. Most of what I know is limited to the first few episodes, fan-fictions, and some searching through the various wikis for certain subjects. I know what Hunters and Huntresses are, as well as Grimm, Aura, and Semblances. I also know that a lot of the weapons in the series are hybrids of guns and melee, and that Dust is used as a power source and as ammunition for some weapons and Semblances. Got it? Okay, that's the disclaimer out of the way, let's move on to the idea.
Maybe some of you have already guessed it from the title, maybe you haven't. That doesn't matter, because today we are talking about an MC with the Semblance version of "Sunshine". Some of you may know what Sunshine is, some may not, regardless, it is the ability which causes one to grow and lose strength with the rise and fall of the Sun. Starting from dawn, the user will begin to rapidly grow in power, hitting his peak at noon, after which his decline starts. There is a bonus ability called "The One" which exhausts all stored strength in order to become the invincible incarnation of power (I'm not entirely sure if there indeed has to be some form of stockpiled energy, I'm just making assumptions based on other peoples' statements). This form is only available at high noon, and lasts for one minute.
In order to make this more like a Semblance, a few changes will have to be made. First off, limiting how much the character physically grows, I don't think many people would appreciate a MC growing to the size of a building every single day. At most I think he should grow slightly taller and more muscular. Second, I think his Aura should not be used up by the Semblance, just increase in both temperature and density. However, if he wants to use The One, he'll have to forfeit all his supply in return for the godly physical strength. Third, it should be similar to Qrow's in that he has no control over it, but he can amplify its effects if necessary (Heat-wise not Boost-wise). Fourth, I don't want him to become completely useless during the night, just not as strong.
Now, who will the main character be? An insert? A Reincarnated individual? Some random man who awoke in a new world? Out of the three, I have to go with the cliche, reincarnation, as it is the easiest to understand how Sunshine got into the world of Remnant. Of course with the vast differences between Semblances, you could easily use the insert. I'm just in favor of using the easiest explanation, which is a wish or granted power.