Sith Inquisitor has a different layout of Companions than its counterpart of Jedi Consular. As a Sith Inquisitor you don't get access to a Companion healer until the planet Hoth, which is at around level 35-ish. Some of the bosses that you will have encountered before then are quite a challenge without a healer Companion. A sith Inquisitor has to rely on being a healer for themself and using a tank companion, or using a Companion like Ashara Zavros and relying on burst damage to take opponents down quick. Playing with a healer Companion is very different than playing with a DPS Companion. The other challenge for Sith Inquistors is that if they play a male character and they want to play around with the Romance options with a Companion, the only female Companion option is Ashara Zavros and she doesn't like dark side choices. She can be bought over with gifts. I highly recommend not trying to romance Khem Val ... could be a little freaky!
I have put together details about the five Sith Inquisitor Companions and all their preferences for gifts right here: