The person claiming self-defence must have the subjective belief that "a threat of force is being made against them or another person." Such belief must also be based on reasonable grounds. They must also act with the subjective purpose of protecting themselves or the other person from that use or threat of force.
This comes from the text of the defence, codified at s. 34 of the Criminal Code. It reads:
34 (1) A person is not guilty of an offence if
(a) they believe on reasonable grounds that force is being used against them or another person or that a threat of force is being made against them or another person;
(b) the act that constitutes the offence is committed for the purpose of defending or protecting themselves or the other person from that use or threat of force; and
(c) the act committed is reasonable in the circumstances.
"Unless the accused subjectively believed that force or a threat thereof was being used against their person or that of another, the defence is unavailable" (R. v. Khill, 2021 SCC 37, para. 52).
The National Judicial Institute's model jury instructions slightly rephrases:
[the accused] believed that force [or the threat of force] was being used against him/her [or against another person] and [accused]’s belief was based on reasonable grounds
Regarding evidence in general: the court needs some evidence on which to give an "air of reality" to every element of this defence in order to place the burden on the Crown to rebut the defence beyond a reasonable doubt. This does not require the accused to testify about their subjective belief. The evidence about the subjective belief can come from elsewhere in the evidence. For example, even if an accused had no memory of the encounter, there may nonetheless be evidence about their subjective belief: video evidence of the encounter, hearsay evidence about what the accused said during the events, physical evidence revealing a defensive posture, etc. The evidence must support an inference of a subjective belief in a threat. It is not enough that the evidence shows that a subjective belief would have been reasonable. But none of that is relevant to the question of what the elements of self-defence are or whether an act taken without the subjective belief that the accused or another person is being threatened is self-defence. The question has helpfully taken the standard approach to legal hypotheticals of just asserting what the facts are in order to take questions of evidence off the table.