Barbarian is the best class for a supporting character without a subclass.
- Most hit points (1d12 hit die).
- Unarmored defence (Start lvl 1 with 18 AC, 16 Dex[+3], 16 Con[+3] and a shield[+2].
- Extra attack at level 5
- Fast movement (10 foot speed increase at level 5).
- Rage - you still get this classes defining feature at level 1 without a subclass.
Compare Ranger, faring much worse without it’s subclass/archetypes; Barring hunter’s colossus slayer or beast master’s animal companion and you have arguably lost Ranger’s most essential features.
Hot take: The most important aspect of a supporting character is their staying alive. Because the most annoying would be having to constantly revive a nerfed PC. So Wizards are out, rangers and warlocks and sorcerers are out, your left with cleric, paladin, fighter, druid.
Fighter is also a good choice with second wind and action surge but its a debatable second fiddle to Barbarian hit die and fast movement.
Losing a druidic circle is harsh but workable. Cleric is fine but boring, but it maybe less exciting to play a party healer if your into that. Paladin flavour-wise could be problematic if you have addedtrouble maintaining their oath etc.Barbarian Barbarian won’t die, is easy to RP and doesn’t-really lose anything of import when compared to other classes.
To make the decision easier, consider if your secondary character were a spell-caster, you will spend time choosing spells and accounting for area effects and concentration. Some people enjoy this aspect of the game and would then be wise to pick a Druid, Cleric or Paladin. By contrast the Barbarian is low maintenance and could still pick up a cure wounds spell and spare the dying cantrip with the magic initiate feat.