Is Master Yi Legend ability a trigger?

No, Master Yi's legend ability is not a triggered ability; it is a passive ability.

According to RiftJudge FAQ #3743 and #6796:

• Passive vs. Triggered: Abilities that use "while" language are passive abilities that are active as long as the condition is met. Triggered abilities use "when" or "at" to indicate they activate in response to a specific event.
• No Chain Interaction: Because it is a passive ability, it does not use the chain and cannot be responded to.
• Immediate Application: As noted in FAQ #5526, the bonus applies immediately when the combat showdown begins (as soon as the defender designation is assigned and the condition of being alone is met), before any action chain begins.

If the condition of "defending alone" is no longer met during the showdown (for example, if another unit is moved into the battlefield), the +2 might bonus is removed immediately.