About Companions

Companions are your faithful allies in your adventure. They'll help you in battle, serve as a mule for taking care of the extra load and basically prevent you from getting all lonely. Some companions have "approval" ratings. These are their overall affinity towards you and is influenced by different actions. Doing the things favorable to the character will increase their affinity while doing the things they dislike will do the opposite. Maxing out a companion's affinity level will grant you their unique perk so it may be a good idea to switch companions once in a while. Take note that the unique perk you'll get from your "maxed" companions will be permanent and you don't need to keep them on your side to enjoy the perk's benefits.

Your first companion, Dogmeat will love you unconditionally and doesn't seem to be affected by your actions. Basically, he's your one true faithful furry friend. The only downside is that Dogmeat doesn't have any unique perks to give you. However, you need to spend a Skill Point since his perk is part of your Perk List.

You can only have one companion at a time. Having a new follower will automatically dismiss your current follower but you'll have the option what location you can send them. If you have multiple workshops, you can place them there to help with the Defense (they don't count towards the population and resource needs).

Equipment

Some companions can have the same equipment as you so it's always a good idea to share any surplus unique/rare equipment and weapon you have in your inventory. This will ensure that you and your partner are always a force to be reckoned with, no matter what threat you're facing.

Romance

If a companion's level is high enough, a Charisma speech check may appear when you're talking to them. Remember, you can save in the middle of the conversation, giving you an infinite amount of attempts to pass that speech check by saving/loading that specific spot. Passing this will elevate your relationship to "romantic" status which will unlock an additional perk called Lover's Embrace. You'll activate this perk by sleeping when a romantic partner is nearby. Your exp gain is boosted by 15% for 12 hours after waking up so sleeping before turning in a quest can definitely help you boost your levels quickly.