I ran into that situation in my PF1 Iron Gods campaign, Iron Gods among Scientists. The party had only three PCs: a half-elf high-Intelligence magus, a strix skald, and a dwarven Experimental Gunsmith gunslinger. The town Torch was recruiting adventuring parties to go into the caves below town to find their missing wizard, and two of the party members were residents of Torch, so the town council was helpful toward them. They offered that they could send a town guard with the party. Really, I as the GM was offering a 1st-level NPC party member, such as a town guard fighter or an acolyte of Brigh cleric. They chose the wizard's daughter, Val Baine, as their 4th party member. I statted her up as a bloodrager to be the party's muscle. I recently converted Val Baine to PF2 rules, so you can see her build at PF1 Bloodrager Val Baine Converted to PF2.
I was lucky that Val had been designed as an extremely sympathetic character. Her plot role was to encourage and offer support to the party's search for her father. An NPC who joins the party as a full member rather than as a follower is automatically a GMPC, and GMPCs can be very annoying. We GMs have to constantly remember that the game is not about our character, that our GMPC is only an NPC who should not steal any glory from the player characters, despite being an equal. I roleplayed Val as an eager assistant who hero-worshipped her more decisive comrades. And I treated her like an NPC, such as having her flatter her way into servitude to protect another party member in a bad random encounter in Lords of Rust. Val missed two game sessions while in captivity, which I would never do to a PC.
And despite Val being designed as a frontliner, she did served that role only at 1st level. We had a magus in the party, after all, who went into melee more often after he aquired his Black Blade at 3rd level. The strix abandoned her plan to use chakrams as thrown weapons and switched to Death from Above flying charges with a lucerne hammer. And at 2nd level the party gained a fourth player who played a human fighter/investigator. Though a PF1 gunslinger is a ranged combatant, they have a very good AC. Therefore, the party had no squishy wizards or rogues, so it did not need a front line to defend the vulnerable party members. The party adopted a skirmishing combat style, always on the move and attacking the enemies from many directions. While half their battles were in cave tunnels of the corridors of crashed spaceships, the other half were outdoors and they took advantage of the space to spread out.
I had to look up Jaethal and Valerie. Since they come from the Kingmaker computer game, I presume that Lorkan is running the PF2 version of the Kingmaker adventure path. I guess that Kingmaker has as much outdoor combat as Iron Gods. Lorkan's party might not need a front line, either.
Neither rangers nor bards are squishy. Traditionally, sorcerers are squishy and need protection, but PF2 lets players build their sorcerers as less squishy. I have a rogue/sorcerer and a full sorcerer in my PF2-converted Ironfang Invasion game, and neither of them is squishy. The rogue/sorcerer often uses his draconic bloodline's dragon claws in melee. The fey-blooded sorcerer runs around the battlefield casting battlefield control spells and healing her teammates and seems to have plenty of hit points, especially since she can heal herself.
For a further complication, Jaethal is an inquisitor and that class does not exist in PF2 yet. Nevertheless, it is fun to imagine an evil undead elf asking to join the party. PF2 does have some undead archetypes for building Jaethal as another class, such as battle oracle. You could give the party the Free Archetype to balance Jaethal's 1st-level undead archetype.
Ask your players if they want an NPC as a fourth party member. However, they might surprise you, as they surprised me, by asking for a different kind of character than you expect.
I am running a 5 person party and I let them bring 1 companion around at all times, under the agreement that the companion gets a split of coin and that encounters get the 6 person adjustment. So far my party has been rotating through the companions, although I think they will likely start to favor Valerie or Tristian for tanking and or healing.