Is utilitarianism foundational to LessWrong?

No. Individual utility calculations are, as a component of decision theory, but decision-theoretic utility and interpersonal-comparison utility are different things with different assumptions.

encourage people to "build character" so as to become more moral beings or improve their behaviour.

This is a solid view, and one of the main ones I take--but I observe that listing out goals and developing training regimens have different purposes and uses.