Flash When Selfless Squire enters the battlefield, prevent all damage that would be dealt to you this turn. Whenever damage that would be dealt to you is prevented, put that many +1/+1 counters on Selfless Squire.
Selfless Squire's first triggered ability prevents all damage that would be dealt to you this turn after the triggered ability resolves, even if Selfless Squire leaves the battlefield.
Selfless Squire's second triggered ability doesn't trigger only when damage is prevented by its first triggered ability's effect. Any effect that uses the word "prevent" will cause it to trigger.
If a prevention effect applies to damage that would be dealt to you while you control a planeswalker, you choose whether to apply that prevention effect before or after the planeswalker redirection effect. If you apply the prevention effect first, the planeswalker redirection effect no longer applies. If you apply the planeswalker redirection effect and your opponent chooses to have the damage dealt to a planeswalker you control, that damage isn't prevented.