A "fair" Summoner with the martial variant... let's look at the Mirror Walker from a slightly different POV.
Chassis: d8 HD, good Will, 2+Int skills on a Cha class (yuck).
Major Features: Spellcasting (includes Cantrips), Eidolon, Summon Monster SLA (includes gate)
-Of these, we seem to be agreed that the Eidolon in some way, shape, or form is the defining class feature. The others can be molded, axed, or adapted at need
Minor Features: Life Link, Bond Senses, Shield Ally, Maker's Call, Transposition, Aspect, Merge Forms, and greater versions of these
- Oddly, most of these seem to work and work WELL with the idea of an Initiating Eidolon.
So, let's set out a theme. The Mirror Walker calls forth an Eidolon which instructs him in the ways of the Martial Tradions. It serves as Sensei and Servant both, and over time they become partners. Neither is truly mighty without the other, but together they are nearly unstoppable.
Spellcasting: Entirely replaced by 6th-level Initiating. Golden Lion, Shattered Mirror, and Veiled Moon (schools up for debate). Charisma is your stat. Recovery is using Aid Another on your Eidolon (they recover by doing the same to you). Summoner gains free Martial Weapon Proficiency of their choice, from any school they have access to.
Summon Monster: SLA replaced by Evolution Surge (progresses in type as you level). Both the Eidolon and Summoner benefit from the evolution(s) applied this way if one is within the other's reach when cast. Also replaces Aspect and greater Aspect
Eidolon: Restricted to the same base form as the Summoner, and must stay the same size and general body shape as its master (so no extra limbs/wings/heads or growing Large/Huge unless the Summoner does so as well). Gains 6th-level Initiating progression in the same three schools as its master (so Martial Traditions would swap on both). Gains proficiency in the same weapon its master chose for free.
Shield Ally: changed to work in both directions.
...I'd play it
EDIT: Incorporating below suggestions, and adding to my personal homebrew book regardless of the outcome. I was against Summoners getting the subsystem, but this is working.