"Guardian Kay'est" is not affected by Spell Cards, just like "The Legendary Fisherman".
Your opponent cannot select "Guardian Kay'est" as the target of an attack.
If "Guardian Kay'est" is your only monster on the field, your opponent can attack your Life Points directly.
As long as "Guardian Kay'est" cannot be selected as the target of an attack, it cannot be targeted by the effect of "Staunch Defender", either.
If "Guardian Kay'est" and another monster are targeted for "Different Dimension Gate", only the other monster is removed from play and returns when "Different Dimension Gate" is destroyed.
If "Guardian Kay'est" and another monster are selected for "Creature Swap"'s effect, "Creature Swap"'s effect is negated because switching is impossible.