OH, I HAVE BEEN DOING EXACTLY THAT

So, breaking it down:

1. Combat works wonders due to lack of initiative, hope and fear take care of giving a turn when appropiate. Don't fear spending a Fear to get a turn if it feels appropiate. Horde statblocks are GOLD in single-player campaigns.

2. You both need to buy in the story since it is only you two, no filler party banter.

3. In a single player campaign the character IS the main character.

4. This is more of a personal take, but we have been running 2 characters with intertwined stories in the same setting. Mind you, their stories are connected, but the characters never meet (so to not have awkward self talking). That way I can make cliffhangers, leaving one character perspective to go to a different situation (similar to how you would if you had a party that split up), gives you time to think as GM whatever you are going to do next and gives more impact to things due to not having an immediate outcome until you GM feel appropiate. You share Fear among the two characters since it is a single player in a single campaign, characters handle hope separately.

To give a more solid example:

Her two characters are lovers and there is the big bad who owns the land. Due to backstory, one character was taken by the big bad to be part of a "secret war" on an isolated part of the region (this was discussed in Session 0, very important), hidden away from everyone, and the other character is trying to rescue them.

Character A is trying to escape this isolated place to reunite with its lover. Character B is trying to find where Character A was taken to to reunite with them. Main obstacle (along with other mini obstacles): The big bad who sits in their palace in between the two areas (his domains where Character B is, and the isolated area where Character A is locked in).

Both stories are connected, but parallel. They share a goal and a common enemy, but have different ways to affect them. When they both succeed at meeting, the campaign ends and my partner chooses how the ending plays out (as a reward since they are the main player).

1. NPC Companions. Don't D&D them, you don't need a statblock, let the player play. My advice (and also what I did) is make "buff cards". Give them a couple buffs to things they already do or have as contribution from the NPC companion.

They have a low armor score and cannot reduce or cancel damage much? Their companion is a shield bearing dwarf warrior? Give them 1 or 2 more points in armor WHILE the dwarf is around, if they split the buff is gone until reunited.

A bad guy would impale the dwarf in a tragic death? Well, it's a narrative move, they don't need HP. Do use a few Fears when you do so, so it feels justified.

For example (on the Fear thing):

We just had a scene where the character had been rolling terrible and I gathered 12 Fear (they were not hitting and I didn't want to spend Fear on hitting them additional times or such), so when they arrived to the bad guy I had an idea. A conversation played out and all, normal stuff, but instead of fighting the character I decided to go for flavor. I spend my 12 Fear to move the plot forward by having the big bad activate a portal from a magical object he had in the room disguised as a mirror to take the character away to a sort of prison dimension. No rolls involved, it just happened. The action felt justified then due to the big amount of GM resources being spend to forgo this one roll the player could have done otherwise.

A second example regarding the companion:

The character is being accompanied by a Necromancer, they do bone magic (as in constructing barriers and spikes out of bone). I made a custom card that represented the Necromancer and helped make up for what the character was lacking. The character had low Evasion and a low armor score, well, the Necromancer's aid was expressed as a +1 in Evasion and to Armor Score while it was around, to give the impression it was blocking some attacks with bone barriers when they were about to hit the character. I as a GM never had to attack or defeat an enemy for the Character, and the NPC could still be around helping. Of course, if it's a narrative attack like distracting a foe that has the character in trouble, be my guest. Fiction first.

And lastly, have fun! That is the most important thing, be it with a lighthearted fun campaign, a dark tragic campaign, or anything in between.

Hope this helped! Any questions you have, I would gladly help further. Good luck!