Sometimes; it's complicated
The echo's movement in relation to other creatures is governed by a slightly obscure set of rules: The echo is not a creature, but it occupies its space, which is an attribute normally only ascribed to creatures. The rules which govern how space is occupied refer explicitly to creatures and movement, and do not map neatly on to non-creatures with no movement speed.
In this answer, I make three assertions about the echo's movement, each of which is less certain than the one preceding:
- The echo cannot move through the space of a hostile creature (unless it's much bigger or smaller than the echo)
- The echo can move through the space of a non-hostile creature (or a very big/small hostile creature) without penalty
- The echo cannot 'end its move' in another creature's space (an imaginative ruling because the echo does not have a 'move' to end)
I'll try to justify these below, but in so doing I'll be making the best of a bad job: the rules for the echo's movement are poorly written, and require imaginative interpretation. I'll illustrate this by going through some of the factors which complicate the ruling.
There are several factors which suggest that the echo's movement might not be inhibited by other creatures:
1. The Echo isn't a creature
The basic rules impose the following restrictions on player movement, which are assumed to extend to all creatures, except in the case of exceptions:
You can move through a nonhostile creature’s space. In contrast, you can move through a hostile creature’s space only if the creature is at least two sizes larger or smaller than you. Remember that another creature's space is difficult terrain for you
Since the echo isn't a creature, it's not immediately obvious that these rules have any bearing on it, or for other creatures wishing to move through its space.
2. The Echo doesn't have a movement speed
None of the rules for movement, including the above, can be applied directly to the echo, because the echo does not 'move' in the way that creatures move. It has no speed of any kind, and moves 'up to 30 feet in any direction' when mentally instructed to do so by the Echo Knight. This movement includes upwards movement and is more akin to a caster moving their Mage Hand than to a creature expending their movement to traverse space in a physical way.
3. The echo is an image
Even if the Echo weren't a creature and didn't have a speed, it might still be constrained by the common sense adjudication that corporeal bodies cannot pass through one another freely. The echo, however is:
a magical, translucent, gray image
One might argue that a 'magical image' can pass through pretty much anything. There's not a great deal of clarity here because the echo has an AC and no resistances/immunities. It's an image you can hit with a stick.
But there is one big restriction on the echo's movement
All of the above is overwritten by the ruling that the echo:
... is the same size as you, and it occupies its space.
'Occupy' and 'space' here are not meant in their most general sense, but are mechanical terms usually reserved for creatures. So whilst the echo is not a creature, it inherits the capacity of creatures to occupy space, and the restrictions of space-occupation which come with them. Whilst these rules refer explicitly to creatures being unable to occupy one another's space, they are extended to the echo because the echo is a space-occupier even though it's not a creature. This is a difficult edge case brought about by poorly written rules.
What does this mean in practise?
Implications of this constraint
The echo cannot move through the space of a hostile creature (unless it's much bigger or smaller)
Creatures, which occupy space,
can move through a Hostile creature’s space only if the creature is at least two sizes larger or smaller than [they are]
Since the echo occupies space, it follows that this restriction is applied to the echo's movement
The echo can move through a non-hostile creature's space (or the space of a much bigger or smaller hostile creature)
This is where it starts to get a little messy. The rules for players state that
another creature’s space is Difficult Terrain for you
Difficult terrain means double movement cost. The echo does not have a speed and does not use speed to move, so 'difficult terrain' is meaningless. Because:
- Space-occupiers can enter the space of another creature
- The echo is has no speed and is thus unaffected by difficult terrain
The echo simply moves through the spaces of passable creatures without penalty
The echo cannot end its movement in another creature's space
This is where it really starts to break down. The echo does not expend movement, and so cannot 'end its movement' anywhere. I would argue here for what I think is the most intuitive ruling: The Echo Knight cannot stop moving their echo in the space of another creature, and is only permitted to move the echo through another creature's space.