Shirou has a Reality Marble because his Origin and elemental alignment were forced into “sword,” which makes his inner world naturally take the shape of weapons and blades.

What that means

A Reality Marble is basically an externalized inner world. In Shirou’s case, that inner world became Unlimited Blade Works because everything about him was tied to swords after Avalon altered him.

Why Shirou specifically

  • Avalon changed Shirou’s affinity to match his Origin, both centered on “sword”.
  • That alignment gave his projection magecraft an unusual, specialized form.
  • He also already had the latent ability; he just lacked the mana and understanding to manifest it cleanly at first.

Why it shows up later

Shirou doesn’t just “get” the Reality Marble from Archer. Archer helps him understand and use what was already his, while Rin’s mana support helps make it possible to deploy it in practice.

In one line

Shirou’s Reality Marble exists because his body, origin, and inner world are all deeply defined by swords , so Unlimited Blade Works is the natural expression of who he is.