It seems like you want a character who is memorable, doesn’t have to keep on rolling death saving throws, and who can actually heal/do their job well. May I suggest something? In D&D, particularly in the Icewind Dale module, there is an item/creature named the Professor Orb. It is essentially a limited AI that you ask questions to about history and the like. You could modify it so that instead of doing history checks, it could heal or otherwise provide support the party. You can also change how it’s personality works, or you could even give it a funny voice! That way, when spoken to, it becomes memorable for the players. I would suggest against it being able to cast offensive spells, however. If you did, it would seriously unbalance the game unless you rolled up stats for it, meaning it could be killed. I hope you like my idea!

First off, I love your username. I've been playing a Githzerai inspired by Dak'kon on and off for the past 2 years and get to say that a lot.

Anyways, it looks like you've got the right idea already about a follower-NPC and that wondrous figurine pet as a reward!

I'm the opposite in that I'm DM'ing for a large group. But I've ran a session where they have had to face a doppelganger of themselves in their minds and the number 1 issue on running 1v1 encounters like that is that they're going to be screwed if they don't roll high in initiative. So the tip there is to reroll initiative after every round.

Next, that's just for a 1v1 scenario. I've only played Mines of Phandelver as a player so I've only got a rough idea of the first arc of the adventure, but that 1st goblin ambush is infamous for outright killing level 1 adventurers rules-as-written if they crit. No tips here, but maybe consider adding at least one more NPC for you or your player to control if you've got at least 3 goblins. Oh and level them up after that encounter.

Last, well what happens if your player gets knocked out? It isn't always the end of the story. The player could wake up in that goblin hideout as a captive and now it's a prison break!

Instead of DMPC I would give your player a retainer or two. Some hireling of less capability and importance than the PC but will complement them. Especially in combat. If they play a wizard they may have a martial bodyguard or two. A paladin a squire and an acolyte there to get some field experience. Give your player full control over these in combat. Outside of combat they could have a thing or two they could assist with. ASSIST with. I mean you want the PC to be the star so emphasize this. Now that I think of it it could be fun to have the hirelings do tasks the PC is horribly ill suited for. As for modifiers I would let the hirelings act at the PC's proficiency bonus without any ability modifiers, or as if they had a 10 in the relevant attribute. This way they won't overshadow the PC in areas the PC is competent in.

Hi there! Glad you saw some of my other stuff!

Here are some articles to help with one on one games:

https://slyflourish.com/running_one-on-one_dnd_games.html

https://slyflourish.com/framework_for_one_on_one_dnd.html

https://slyflourish.com/balancing_combat_for_one_on_one.html

Hope that helps!

Obvious but less helpful answer: DND is designed for parties. Play a different rpg if you don't have a party. Fate is very good and more in line with how new players try to play RPGs anyway. It is not nearly so hard to learn new games as you probably think it is. DND is a weird idiosyncratic mess and that's why it was hard to learn.

More helpful answer: give them a sidekick or two. I think Tasha's has rules for it, or you can just wing it. Don't make a full dmpc. It's still going to suck because DND is inflexible and not designed for this.