Echo Knight says "when you take the Attack action" you can have it come from your Echo's spot, so it must be an attack that uses the attack action, and not a spell that uses the cast a spell action (which even booming blade and green flame blade do). A DM might let you still use your Echo, but it's not RAW. Further, BB and GFB state in the description that you're doing a melee attack against someone "within five feet of you" so it won't work to have the Echo use it against someone who's not within 5 feet of you.
As others have pointed out the first requires that you take the Attack Action. In general this will mean you cannot use this to cast a spell. However a Bladesinger's Extra Attack feature let's you replace one of your attacks with with a cantrip. So the specific combination of a Bladesinger/Echo Knight can potentially cast one cantrip on their turn from the echo's position.
The second way an Echo Knight can act as if they they are in their echo's space is if an enemy adjacent to the echo moves at least 5 feet away from it. When this happens the Echo Knight can make a opportunity attack as of they are in their echo's space. Opportunity attacks are normally limited to melee weapon attacks but the War Caster feat lets you cast a spell instead of making an opportunity attack so long as the spell only targets the creature that provided the opportunity attack.
Those are the two ways I could think of that might let this happen. However it will depend on how your DM rules on how these specific features interact.
Given your multiclassed with a spellcaster or have gained spells some other way, can you cast spells through your Echo?
Ruling stats you can choose to have your attacks originate from yourself or your Echo but I'd assume the same for spells...
thoughts?
DruidVSAdventure
Check out my Homebrew Class The Evoker
Echo Knight says "when you take the Attack action" you can have it come from your Echo's spot, so it must be an attack that uses the attack action, and not a spell that uses the cast a spell action (which even booming blade and green flame blade do). A DM might let you still use your Echo, but it's not RAW. Further, BB and GFB state in the description that you're doing a melee attack against someone "within five feet of you" so it won't work to have the Echo use it against someone who's not within 5 feet of you.
Birgit | Shifter | Sorcerer | Dragonlords
Shayone | Hobgoblin | Sorcerer | Netherdeep
Manifest Echo (https://www.dndbeyond.com/sources/egtw/character-options-subclasses#ManifestEcho) has two different ways you can act as if you are in your echo's space.
As others have pointed out the first requires that you take the Attack Action. In general this will mean you cannot use this to cast a spell. However a Bladesinger's Extra Attack feature let's you replace one of your attacks with with a cantrip. So the specific combination of a Bladesinger/Echo Knight can potentially cast one cantrip on their turn from the echo's position.
The second way an Echo Knight can act as if they they are in their echo's space is if an enemy adjacent to the echo moves at least 5 feet away from it. When this happens the Echo Knight can make a opportunity attack as of they are in their echo's space. Opportunity attacks are normally limited to melee weapon attacks but the War Caster feat lets you cast a spell instead of making an opportunity attack so long as the spell only targets the creature that provided the opportunity attack.
Those are the two ways I could think of that might let this happen. However it will depend on how your DM rules on how these specific features interact.