Tezzeret the Seeker
Tezzeret the Seeker is a planeswalker who's effects need loyalty counters to use. The number of loyalty counters also means how much damage he can take before he is destroyed. You cannot have more than one Tezzeret on the field at any time. Planeswalkers can only be targeted as you would a Creature or Player, however this card isn't destroyed by world affects from cards like Wrath of God or Mass Calcify.
Tezzeret is also one of those lesser used planeswalkers because his abilities are less than impressive. However, his first and second ability are very good. The first allows you to untap 2 artifacts, which if you're running the artifact lands from Mirrodin, then that allows you to use two more lands! It can also untap your artifacts and artifact creatures! His second ability allows you to search for artifacts with X converted mana cost. If you know your deck well enough, you can basically search for anything! His last ability is decent, but not really used. Even though they all become 5/5's, the only addition they get is that they can attack, which can help or harm you.