I could write several essays on this but I think an overlooked key element of why Light never wears the mask for Ryuk is that at their first meeting - after the initial jumpscare which couldn’t really have been avoided - the first strong emotional association Light forms in relation to Ryuk is relief.
Light’s just spent a week waiting for a shinigami to show up and be mad at him, on top of everything else he’s dealing with. He’s expecting to be punished in horrible supernatural ways for what he’s done. And what he gets is Ryuk, who hits him in rapid sequence with “I’m not here to kill/hurt you”, “I won’t take your new toy from you”, “there’s no price beyond an emotional one for using the Death Note, the only real consequences will come when you die and they’re not even worse than anyone else gets” and, importantly, “omg let me look wow that’s literally the best anyone’s ever done it!”
If you look at what’s actually being said and not at the theatrical scary posturing (which Ryuk is visibly struggling to maintain anyway, he keeps dropping out of Scary Mode and into his Domesticated House-Shinigami persona in that scene and then catching himself) - Ryuk basically lovebombed Light to heck in that first meeting. He reassures Light on every single point that’s been eating him alive with panic for the last five days. He praises, admires, even opens up and offers a bit of personal honesty and secret lore (“the magical realm of the death gods actually kinda sucks and I was bored”) - to which Light responds with a flood of confessional honesty of his own. Which Ryuk then meets in turn with total acceptance (just a calm “I see”, after like seventeen pages of “and I’ve done all this!” from Light) and zero judgement.
In other words, at that first meeting, Ryuk offers Light exactly what Light most wants in that moment: attention, comfort, reassurance, enablement. Is there any wonder Light falls headlong into that and never even tries to climb back out?
(Oh, and yeah, what Ryuk did at the end was a mercy kill for sure. If Ryuk couldn’t intervene to help Light (which if he loved Light at all we know he couldn’t) and hadn’t intervened to kill him, Light’s future would’ve been a lifetime of imprisonment - and if Nia destroyed the notebooks, he’d be without his memories and with no way to get them back. Exactly how awful would that have been for him, not even knowing what he’d done and likely unable to believe it of himself even if Nia had told him he was Kira? The part-one imprisonment arc all over again for the next forty or fifty years, without even the hope of escape?
Yeah. Forty seconds of terror and a heart attack at Ryuk’s feet was definitely merciful by comparison.)
