Yes, forms as in (mouse form / creater form / beautiful concubine form / fire storm form etc).
The ally spirit may have only one of the following powers: Inhabitation (p. 195), Materialization (p. 398, SR5), or Possession (p. 197).
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Of course I can speal only from the point of view of a mage started arouind 10 years go in 2003 or 2004, which is now around 800 Karma. I started building up the Karma when I reached 400 (or something around that) and summoned my ally with around 500 Karma with force 7. Mind you, that was in SR4, but the rules are the same. They are pricey and you will have to experiment what your gamemaster and your other co-playes will accept, considering that youi need double time to solve actions for the ally and for your mage.
Consider the following for a force 6 ally with arount 100 Karma
1) unlimited services
2) forms like "cat" and "summoner" ("why yes, officer I was not at the crim scene, I was 50 miles to the south in a restaurant when this terrible crime happened, you can check with the waitress there, I belive Weny was her name2)
3) +6 to all spellcasting tests (compared to a36 Karma for a force 6 power focus)
4) +6 spell defense
5) 12 hard armor (definately not a front line fighter)
6) 2-8 Edge (from the summoner, which is currently the only spirit type availble for player characters with their own edge attribute, effectively doubling the edge for magic operations)
7) unlimited use of the spirit powers accident, concealment, elemental attack (lightning), energy aura (for close combat), Guard, Influence (instand mind control), movement with a speed increase of x6, search and weather control (come on, you are a shadowrunner, you are usually there where you should not be, unlimited movement, concealment and point point fog/raid/heat for infiltration purposes? That´s ninja-heaven right here).

The spirit is getting far better used to the modern world (personal interpretation) because he is far more bound to it depending on the tradition. That old man / young girl over there driving a bike or playing chess game? It might by an ally spirit..
9) A loyal servant incapable of treachery (if you are in good standing with your ally spirit).
10) Theoretically unlimited spell sustaining while not usable in combat (+4 willpower, +4 drain attribute, +4 reaction, +4 intuition, +8 increase reflexes) - just imagine the nuyen and Karma costs for spell sustaining foci. And yes, this is borderline rule abuse (or way beyond it - check with your game master).
11) The ally can get skill you have only at 1. Heavy Weapons? Alchemy? Driving a car? Speaking that obscure language?
12) Double spells, either normal spells, innate spells or spirit powers. Like Levitate for the group. Invisibility, stun bolt, lightning ball.
13) role play, flair, style. I am a big Fate/zero fan, so some may imagine what my ally is. This is hard to quantify, if you have a creative gamemaster it can be a cool experience, if your gamemaster hates magic or spirits or cannot cope with the added complexity you are not doing yourself a favor.
For 100 Karma you get 20 spells, 3-4 initiation grades (with metamagics), 2-3 attribute increases like magic, and 2-3 skill from zero to 6 or 1-2 to 10, or any combination of it. Very powerful of course, and yes the costs are this high. As I said it depends on the campaign, the Karma rewards and your gamemaster on how he interprets the usage of some rules.
Definately not a trap, but an option to consider depoending on your campaign.
Personal recommendation: if you are in a long compaign going over the 400 Karma points: check with your gamemaster and go for the ally spirit.
SYL