* Crusader of Endymion's effect targets a face-up monster you control. This means the effect can only be used on your own monsters.
* Spell Counters are not a card type that can be placed on an opponent's card. Spell Counters are usually placed on monster cards, and their effects are typically tied to the card they are on.
To put a spell counter on an opponent's card, you would need a card with an effect that specifically allows you to do so. This is a rare effect and would likely have restrictions on which cards you can target.
It's important to always check the card text carefully to understand its effects and limitations.