Does an insurer have to defend me if the lawsuit includes even one covered allegation in Ontario?

Generally, yes. Ontario courts assess the duty to defend by looking at the substance of the allegations in the statement of claim against you, and if even one of those allegations, if proven true, could fall within your policy's coverage, the insurer typically must defend the entire lawsuit - including the parts of the claim that clearly fall outside coverage - until the coverage question is resolved some other way. The rationale is practical: your defence against a multi-part claim isn't easily split into "covered" and "uncovered" pieces litigated separately, so the insurer takes on the whole file once any part of it could trigger coverage.

This doesn't mean the insurer ultimately has to pay for every part of the claim if you lose - the duty to indemnify at the end is narrower and depends on what's actually proven. It also doesn't stop an insurer from later challenging coverage through a reservation of rights or a separate coverage action while still funding your defence in the meantime. If your insurer is refusing to defend based on how they've characterized the claim against you, it's worth having the actual pleadings reviewed, since insurers sometimes read them more narrowly than the law requires.