So you've got a few adjectives for the character, to start off with. Some people are a fan of the figure-your-character-out-as-you-write approach, and some prefer outlining the character first. Since this is an assignment, and you don't want the character who spontaneously appeared as you wrote to differ from the one described in the assignment, I'd have to recommend the latter approach.
What makes a "good" or "lively" character:
* Details! What are his quirks? Does he wrinkle his nose when something disgusts him, or wipe his right hand on his pants?
* What does the world look from his point of view? What's his job? Does he like it? Any best friends? How'd he grow up? Since he's smart, insightful, and thoughtful, how did this shape his philosophy? You need to know to write him well, and we as readers find difficulty relating to him unless we can get inside his head a little. This will also be crucial for your self-description.
* Give him flaws. Up until now he seems a little <PERSON>-ish. He's smart, thoughtful, insightful, clever--he's most likely gone far in whatever he's done. Reserved and mysterious? Even better, the girls are swarming and we're all jealous of him and wondering what he's up to. 425
Now balance that out. Is he arrogant because of this? Or is he eager to make people like him and bends over backwards for them? Or does he not have this success at all and is angry at the world for not giving him what he thinks he ought to get? This and the point above will most likely determine how he reacts to others.
* Last but not least: We don't see his biography, we see him in action. Specific to your situation it seems like you're forced to give a little biography, but my general advice would be not to make it too long, and intersperse the rest of your biography in with the action, if you need to add it at all.
You've got a character, you know how he thinks, now it's time to throw him in a situation. Roll dice if you have to: 1 for getting mugged, 2 for being fired, 3 for meeting with an ex, so forth and so on. And build the situation such so that you can show most of his character traits in the situation. Think about what he might do to show the qualities you want to highlight.
For instance getting mugged: Your main character stops/talks down the attacker, maybe makes some comment about the mugger's motivation that obviously hits the nail on the head (because he's smart and insightful) , and lets him get away with a "dummy wallet" with only a few dollars (clever, well-prepared) and continues on his way. This scene is observed by an acquaintance who is following the main character and wondering what he's doing in that part of town at night, without having told anyone (mysterious and reserved) . 425