SpongeCob

basically their personalities in a nutshell: they build things

Phineas: Cheerful kid who's nice to everyone and always looks at the brighter side of things. Extremely creative and has a major tendency to express his creativity. Isabella has a crush on him, but he's absolutely oblivious about that.

Ferb: A British kid who doesn't talk much like Phineas, but is a "man of action" regardless. Just as filled with creativity and has a knack for making large-scale projects like his stepbrother.

Here's my two cents. And if you still think they're boring characters with no defining personality traits beside "they build things," I could at least say that there are few moments where Phineas gets angry at someone, like Candace in "Mission Marvel" for example.

Phineas and Ferb may not be the most interesting couple of characters in the aptly named series, but I don't think they are anywhere near being bland.