I generally prefer to give any ally spirit a few forms.
1) Their default "spirit form", in your case the flaming samurai.
2) An innocuous metahuman form with street cloths, usually someone you can pass off as your "companion."
3) Said companion in more formal wear, for those Johnson meets in ritzy restaraunts and times where you need to pass for a corp type.
4) An animal form, such as a dog or a raccoon, for scouting missions where a physical presence is still required.
5) Something really, really intimidating, such as the kind of thing that gives H P Lovecraft nightmares.
In your specific situation:
2 & 3 would be a human/metahuman with dusky skin and red hair, as mentioned earlier.
4 could be any kind of animal with reddish fur, or associated with fire.
5 could be a number of things. One is the appearance of a swirling, sentient fire a few feet across... just imagine this roiling mass of flame chasing you down the hall.
A standard Fire Elemental form works also, and could help trick enemy mages. ("Why the hell isn't my banishing working? It must be stronger than I thought!")
Perhaps a "magma golem" form? An eight-foot-tall humanoid form made of crackling, steaming lava coming at the sec guards...
Remember, the spirit's forms are mostly for your benefit, and for fooling anyone who sees them. Trick them into thinking your spirit is a cigarette lighter, or that you can summon red dragons!