Injection from Proper Class to Class
Theorem
Let $A$ be a class.
Let $P$ be a proper class.
Let $f: P \to A$ be an injection.
Then $A$ is proper.
Proof
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Aiming for a contradiction, suppose $A$ is not proper.
Then it must be a set.
By Injection to Image is Bijection, it follows that the restriction $f \restriction_{P \times f \sqbrk P}$ is a bijection.
By the corollary of Class is Proper iff Bijection from Class to Proper Class, $f \sqbrk P$ is proper.
But since $f \sqbrk P \subseteq A$, this contradicts Subclass of Set is Set.
And so by contradiction, $A$ cannot be a set.
Therefore $A$ is proper.
$\blacksquare$