Well, in the story I am writing at the moment, I use a new species of creature. Anyway, my main two characters (of this species) are serious, and the stuff which happens to them pretty dark/evil, but I also think I have space for a younger character. His would be a side story on the side of the main story around the other two, and I was just wondering because this younger character is still at school, is it a bad idea to mix the serious aspect with a lighter, more teenage-highschool side story? This character would also be in love (with a human), the traditional teenage romance kind of thing. My motivation for using the young character is to show a more human side to this species I have created, and to give a exterior perspective on the events that unfold concerning the other two. I was wondering if anyone has seen anything similar done before? Or has any advice on how to make the two storys combine well.
Stay away from cliches; you may be delving into realms of the cheesy. I think you realize this, though, which will help you to stay away from any 'blah' story lines. It is very good to develop a variety of characters, because it will bring energy to your story. Just try to stay away from the same olds and the stereotype characters. Trying to create multiple perspectives is great, and can take a lot of tricky maneuvering, but (if done right) it will definitely improve your story.
A comic relief character still needs to interact closely with the principal storyline. That character should not be there simply for lightening mood - the interaction with the other characters and events should advance the plot in a material way. Perhaps that character's antics land him in trouble that complicates the other characters' ability to resolve the main conflict, even places him in jeopardy that they must try to rescue him from. In that way, the comic relief character can also learn a valuable life lesson that develops his character, too.
I'm curious about why you can't reveal the more "human" (so to speak) side of the two other characters through their own actions...? If they're developed enough as characters, you might not need the younger character to show the lighter side of things. Even the most serious people have their lighter moments...at least, if they're not cardboard cutouts. An otherwise serious character having such a light moment illuminates this side of their character even more, and the point is much better made. It makes them more rounded, believable characters. I'm not saying you shouldn't include the younger character, just don't include him for the sole purpose of making this species more sympathetic. You should be able to do that with the other two characters. If they need to rely on the younger character to show their more "human" side, then the two primary characters aren't that well developed. Readers still won't sympathize with them even if the younger character tags along. And to agree with the above, provide comic relief only as a means of supplementing the main part of the story, not in spite of it. Humor shouldn't be there just for humor's sake--it should serve a bigger purpose or else it will feel forced. I don't think that's going to be an issue, since your main goal is to show the more human side of this species--which humor can do--but again, please take note of what I already pointed out regarding using one character to make two other characters look sympathetic. It won't work. They need to look sympathetic on their own. The younger character can CONTRAST with the otherwise serious older characters, yes, but all their humanity shouldn't be wrapped up in him alone.
Once, I was in a group of friends speaking Spanish. My best friend's boyfriend was there too, but he didn't speak Spanish. Instead, he was looking up stuff online. Our conversation turned to someone's family in Spain, who had been killed in a bombing. At that moment, the boyfriend started laughing uproariously. We all shot him the most unpleasant looks! It wasn't his fault, though; he couldn't understand what we were saying. It's just that the humor contrasted very badly with the topic of discussion. Now, imagine a story in which something dark and terrible happens to a main character, but the next passage is about prom woes or a pimple in omigodlikethecenterofmyforehead! Each, in its proper context is fine, but juxtaposed, the change can be really jarring and unpleasant. My point is this--if you change tone abruptly in a story, it has an effect on the reader. Just make sure it's a good one.
I think that you should only fit the "comic relief" into the plot if it works. If it seems too cliche, and if the readers will surely notice the cheesiness of it, then I think that you shouldn't bother to add that character. Not every story needs a comic relief, in my opinion. If you really do want to fit in this character, then just really try to make it seem completely natural and not like "um here's a character he's the comic relief"...well, not exactly like that, but I hope you get what I'm trying to say =)
Forgive me if I missed something, I kinda rather skimmed through, but what you are describing (should be) called foil - and there shouldn't be any problems.
A foil is a character whose personality is designed in such a way that it will emphasize a particular aspect of another character's personality. It's kind of like putting an elephant next to a mouse to show how big the elephant is.
