Fighter is more offensive, having better weapon proficiency but their protect an ally only comes at lvl 6.
Champion is more defensive, having better armor proficiency and their protect an ally is online at lvl 1 and they have a healing focus spell.
The other options are usually casters.
Cleric with the protection domain could works as defenders and they have sanctuary and forbidden ward that are in the theme too.
Abjuration Wizard Focus spells and use of spells like walls and Resilient Sphere.
Bard Inspire Defense and spells like sanctuary, Resilient Sphere and forbidden ward.