Hi, Icetama4525;
Like pretty much everyone else has said before, generally "bad characters" are caused by a lack of proper care and whatnot. It would indeed help to have a version number, but I have a few general tips that you could try to get that good character you're after:
-How often do you feed you Tamagotchi? Having dealt with pretty much every version except V2 and V5.5, I can safely say that feeding is a huge factor in your Tamagotchi's adult outcome. If you don't feed it much or in a timely manner, chances are it'll go "bad." Likewise, don't feed it too much. (I've gotten many a Kuchipatchi doing this, but I suppose it's kinda debatable if he's really a "bad character," since he's a pretty popular Tamagotchi.)
-Play with your Tamagotchi a good amount. Don't ignore its happiness meter, and try not to use snacks whenever possible. (Again, this can cause a Tamagotchi character to go bad. In some versions- the Gameboy Tamagotchi and older toy Tamagotchi versions are especially prone, I tend to find- snacks can sicken and even directly kill your Tamagotchi! I'll admit that I've done this, and chances are you've possibly done it too at one point to save time.) It's not that hard to play with your Tamagotchi to raise its happiness. After all, you bought it to play with it, right? Also, notably, on most versions, playing tends to reduce weight, which can, again, influence your Tamagotchi. The optimal weight range your Tamagotchi should fall in tends to be generic, as weight usually shifts with Tamagotchi evolution, but can matter especially on newer Tama models.
-Check on your Tamagotchi often. It's crazy, but it really doesn't take long for your Tamagotchi to randomly get sick, get depleted stats or whatnot, and the longer it's sick, the worse off it'll be. I've done this several times, and it's surprisingly easy to do.
These are usually the general steps I follow when raising a Tamagotchi, or even any virtual pet for that matter. So yes, Tamagotchi are relatively easy to care for, but to master a good character requires just a bit more effort. Chances are, you've either neglected or taken too much care to follow one or more of these steps. And don't worry- you're not a bad owner just because of this. It's very common. Tamagotchis are about balance, just like pets in real life. Also, don't forget, every stage in a Tamagotchi's development is important. Don't care for your Babytchi around the clock and simply leave it for too long the minute it turns into a Tamatchi or whatnot. How you treat it, stage by stage, directly influences the outcome.
Also, be sure to check out the version number on your Tamagotchi, especially things related to care and character charts. You might get a bit more specific information.
~nicki9