I want to play some kind of Inquisitor but I am unfamiliar with this setting...
Inquisitor is a good class. I'm not an expert on optimization but I know how to make a solid, fun to play character. I'll try to help.
]I want to go Necromancy on the magic if possible.
I think you'll feel a bit let down by that character to be honest. First of all you don't get very many necromancy spells as an inquisitor. You get some cool stuff. Some very cool stuff. But it's a mix of everything so having a theme to your magic doesn't really work. Unless you go with enchantment, but that can be frustrating a lot of the time due to immunities. You need a maxed out stat + best WIS item + 2x spell focus and items to boost DCs before it stops sucking. Even then many things are immune. I'd suggest going with an eclectic mix of magic used for healing, protectives and support and dealing big damage with your bow. If you want to be a necromantic archer, cleric (ecclesitheurge) is better because it's got a better spell list (later on. Early game isn't great but it gets amazing). Your archery wouldn't be as good but your necromancy would be much, much more satisfying at high levels. A cheeky one level traditional monk dip could give you AC too, if you're into that kind of thing. But you don't need it if you mainly play turn based and being behind on spell progression is gay. I say keep it single class.
I tend to avoid combining archery with DC based casting (all the inquisitor necromancy spells haves saves afaik) just because archery needs STR for damage, DEX to hit and WIS for spells per day and spell DCs. That being said judgements help with a lot. At high levels judgement justice can make up for a middling DEX score, and judgement destruction + your bane weapon will help out with your damage. You'll want to have high DEX and wear items to boost your STR to get your damage up as priority. DEX items would be secondary because your judgements and feats should get the job done there, at least on hard DIFF.
Here's something I put together just now. Never play tested this so don't take it uncritically. This is just to give you an idea of some of the feats you may want to take.
Race: Aasimar - Plumekith (Garuda blooded) gets +2 to DEX and +2 to WIS. It's a good choice for an archer/caster. Human is good for everything. I'd suggest not playing against your racial strengths. You just end up wishing you were stronger. So pick something that fits.
Stats (For stinky DC casting build)
STR 12
DEX 18
CON 12
INT 7
WIS 19 (All points on levelup here)
CHA 12
Stats (For awesome ranged combat with buffs and healing build)
STR 16
DEX 17 (All points go here)
CON 14
INT 7
WIS 16
CHA 12
I left some CHA in there because you have an iquisitor class feature that adds your class level to intimidate checks.
If you decide to go cleric dump CHA for more of something else (Like DEX or more CON) and go with 19 WIS increasing to 24. Forget about channel energy (it's poo in the base game) and forget persuasion too because you don't have enough stats and feats to run three things (with modded domains though, you can do a lot more).
Anyway, you said inquisitor. The poor man's cleric (JK. Sorta). So lets go back to that. You get three skills with this character. I went trickery, perception and persuasion. Perception is really good for an inquisitor, because of the high WIS. If you want to do more archery than magic your character would probably be more satisfying to play. Inquisitors are better martials than mages TBH, but they can be a load of fun to play. Never done one ranged but I am playing a melee crit build inquisitor right now in beneath the stolen lands and he is epic. All the skills are useful in story mode and all are class skills for your inquisitor. Pick 3 that use DEX or WIS.
Possible feat progression (check AB requirements yourself to make sure this works. It should).
1. Point blank shot
3. Precise shot
5. Weapon focus longbow or spell focus necromancy (enchantment gets more spells for this class).
7. Rapid shot
9. Clustered shots
11. Manyshot
13. Improved critical - Longbow
15. Hammer the gap (good for archers and better at high levels. A nice boost to damage). Greater spell focus is mandatory if you're going DCs. You kind of do have to choose which you will be best at. Your judgements will mean you can still hit with your bow if you go for DCs, but if you don't put everything into your DCs your offensive magic will be all but useless. I'd still go for combat skills in this class, because of the spell list.
17. Optional. You could take a metamagic. I like quicken spell for even more clickable round/level buffs in combat. You could also empower or maximize your boneshaker to give you more offensive spell output, or heighten your favorite save or suck enchantment spells.
Spell penetration is also an option but you have a judgement that helps with that. My choice however would be critical focus. I love that feat. It often gets overlooked because it does its work in the background but I just took it on my rapier using inquisitor and holy shit he's critting like crazy now. Quickened true strike or quickened divine favor are tempting, but I think I like crits more. It's a close thing though.
19. If you're lucky enough to reach this level just take anything. The game is basically over anyway. Take something that sounds fun to you, but that you will be ok with missing if you finish the game before reaching this level.
If you decide to go with more archery (you should) then drop the spell focuses entirely. Extra bane weapon (I think that feat is in the base game) and critical focus. I prefer rapid shot to deadly aim but opinions vary on that one. I like hitting stuff over missing for big damage. But there is an item that gives you +4 to hit with ranged. If you have the DLC you will find it from a boss. You don't need to look for it. So maybe when you get that you could take deadly aim? Yeah that would work I think.
Is there a better Inquisitor companion that will make me regret doing this build?
Well she's not an archer but kinda... yeah. She's a necromancy themed inquisitor in a way your PC can never be. Without spoiling too much yeah, she's a better necromancer than you (still the wrong class to be a great one though). You don't have to use her though. There are lots of companions and you pick the ones you like best to make up your party.