
Chell Raighn |

I'd probably consider it a tumor familiar since it shares many of the same mechanics, and thus not able to be a protector.
The reason that a tumor familiar can’t be a protector is largely due to its fast healing… a protector tumor familiar could effectively make an alchemist or aberrant bloodline sorcerer near impossible to kill… the nature bonded magus on the otherhand doesn’t have any such feature to its familiar… however… I must point out that while yes, the magus controls when the shield master ability is activated… it has no effect while their familiar is merged with them since while merged a nature bonded magus’ familiar is incapable of taking any damage from attacks or effects… shield master is an effect and requires that the familiar and master take an evenly split amount of damage. So if the familiar simply can’t take any damage the ability fails. It would be different if the familiar simply had a crazy high amount of DR, but flat immunity to damage from all sources renders shield master unusable.