Companions come in two varieties, at the moment, droids and beasts. We will break down the similarities and differences below, as well as going over the mechanics and known bugs of each.
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Droids are different than Beasts in that they require no perks or skill investment at all to command and use. Instead, these are crafted by engineers and come in a great variety of builds and abilities. You can find the specifics of their parts, how they effect stats, and the abilities they can use via programming Instruction Discs in the Droid Recipes section of the Engineering page. Summonable NPC Droids act, more or less, like an extra player controlled by an AI.
Items placed in droid inventory can break and become unstackable. Splitting this stack creates two unstackable stacks. Be wary of putting things on your droid you wish to stack.
Customizing an NPC Droid's colors will change them, irreversibly, to silver only. There is no way to change them to any colors except the default purple and yellow or silver, as altering any of those choices turns them permanently silver. Likewise, their armor suffers the same fate, though there is a workaround explained below.
Droid armor, once customized, must be removed from their inventory and placed into your own, then put back into their inventory, and then re-equipped to them, or else the armor will always revert to its last appearance when the droid is re-summoned.
There is a workaround to coloring your NPC droid's armor which requires a player Droid. Remove the armor from your droid and give it to the player. The player must then equip it and color customize it before handing it back to you. Then, you can equip it to your droid and it should retain the colors. Do not try to alter or edit the colors again, or they will once more set themselves to silver. Unfortunately, there is no way to make the droid's head match the armor colors. They will always be default purple and gold, if unaltered, or silver.
One bug that persists across both droids and beasts is that they will attack friendlies who deal damage to them with AoEs, OR who trigger Provoke during combat next to them. Using droids or beasts means not using AoEs. Their aggro can be reset by transitioning areas or dying and being revived.
Beasts require a skill investment and perks, but not months of grinding materials or millions of credits to create. Beasts come in a wide variety of options. The majority of non-sentient creatures across the landscapes of the planets of SWLOR can be tamed and come with various stats. A list of those creatures and their stats can be found here! Beast Mastery perks can be found here!
Beasts can also be fed Pet Food to allow them to gain temporary stat bonuses. Another difference is that beasts gain experience and can gain levels and their own perks, offering perk variety. Unlike droids, which need Droid Assembly Rank 5 to have their programmed instruction discs changed, Beasts can have their perks and skills retrained from beast stable terminals. From these terminals you can check their stats, their Purities, their perks, change their name, switch your currently active pet, or release them.
Kinrath seem to sometimes break and instantly die upon being summoned, still using up your cooldown timer. If this happens a DM will have to assist you or you can simply release the Kinrath and tame another pet (this will obviously lose all progress on your pet, so be warned).
One bug that persists across both droids and beasts is that they will attack friendlies who deal damage to them with AoEs, OR who trigger Provoke during combat next to them. Using droids or beasts means not using AoEs. Their aggro can be reset by transitioning areas or dying and being revived.