Who was the other person the colossal titan ate?
This was an unnamed, unlucky poor individual who was at the wrong place at the wrong time. If you watch this Youtube video, they aren't entirely sure who the Colossal Titan ate either.

It got Ymir!
...
And they got somebody else, too!

Also, while reading this reddit post, people say the Colossal Titan ate him to steal his ODM Gear

Just a random person. They didn't care about the guy, only about stealing his gear.

This is confirmed by seeing the Colossal Titan wearing the ODM gear in it's human form in the scene when they were camping out in the forest