This question occurred to me after reading another one: Can the Echo Knight's Echo make Ability Checks?
That question asks whether the Echo Knight fighter's echo can make ability checks. A grapple is an attack, which the rules permit the knight to make from the echo's space, and is an opposed skill check between the creature being grappled and the knight (not the echo). So what happens when the grapple is successful?
The rule for the grappled condition states that:
The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.
The problem is that the Echo Knight is never moved out of reach by an effect, because it was never within reach to begin with. The rules say the attack can "originates from [...] the echo's space". That's not the same thing as being within reach.
Or is it? The rules for Manifest Echo (EGtW, p. 183) effectively grant the knight reach from that space because the knight can make attacks of opportunity from the echo's space.
So what happens after an Echo Knight successfully grapples a target that is within reach from the echo's space, but not from the knight's?