While Tiamat serves as an introduction to the concept of Primal Beasts, Colossus and his role as the Primal Beast of the Valtz Duchy go more into their history; he is used to introduce some more critical lore of the setting, principally that mortals and Astrals, a mysterious race with higher powers, fought a terrifying war a very long time ago, and that the Primal Beasts were originally created as weapons. They're not natural lifeforms, but rather, artificial constructs left over from a conflict a millenia ago. This explains the wide variety of forms they come in- from mindbreaking fifth dimensional abstractions to fifty foot tall angel twinks. GBF's deeper lore is a hell of a ride.
As for Colossus himself? I've never been too particularly fond of him and think his design is a little generic; he's a big hunk of flamin' armor with a great big sword. There are a lot of jrpg designs like that. Sorry, king. It's not really his fault that he doesn't stick out to me much; again, the first couple Primal Beasts we're introduced to are largely designed to sort of help us understand what exactly they are. They're Primal Beasts for beginners. It's like how in Elder Scrolls Oblivion they start you off with the Healing spell and the Flare spell, before you go off and learn the more advanced magics.
Oh actually I take back anything mean I've ever said. This is adorable. I love Colossus. He's just big and chunky and shaped like a friend. Look at his lttle eyes.
Final Score: 2.5/5 Big Chonks