Either you buy roster space from the in-game shop or heed @Rduke77’s advise (mine deviates a bit from his):
Rigard+CB - I have 4 maxed: 1 at +18 defense>health costume version, 1 at +18 defense>health original version, 2 unemblemed and only at 4/01 on both original version. The last 2 are not used.
Sabina - I have one maxed. Recently got the costume and will level and max it. Plan to get a second copy to max the costume version.
Khiona - I don’t have her, but believe me, when she was first released in 2018, I badly wanted her since I was lacking in purples and my tabards were already accumulating. Good thing Kage, Seshat and Sartana came to me on that same year.
Proteus - I have 2 maxed. Barely used anymore except on certain events.
c-Joon - I have one maxed, both the original and costumed version. I have a second Joon, both versions at 3/70 and may be investing some of my 21 darts on my 2nd Joon. I just need to finish placing Justice to 3/70 first, followed by Owl to also 3/70.
Leonidas - I have 1 at 3/70. I still have 1 copy of him untouched but may be leveled and maxed 2 copies of him if I get his costume
Gullinbursti - This pig is good. A better substitute to Vivica without costume since both are slow but with the pig, maxing him is cheaper and a lot easier. Not only does he heal, he overheals as well as dealing damage to all. My first pig is maxed and emblemed to +19, second one maxed but unemblemed, and maxing the third currently at 4/66. All of them will be very useful in ninja tower, as well as wars and other events.
Jackal - I have 5, only managed to max 3, 2 highly emblemed at +20 (attack nodes) and +19 (defense nodes). Will be working on 4th Jackal soon, maybe after working on Kailani costume.
Onatel - I have only 1 maxed at +18. If I get another one, I may level it but up to 3/70.
c-Magni - I have 2 Magnis at 3/70. May plan to invest scopes on him if I get his costume. Not really a priority.
c-Sonya - I have 2, original version maxed with MCB, the other one only the costume is maxed while its original is only at 4/01. No emblems.
Glenda - I have 4. One is already at 3/70, second one at 2/60. Originally planned to get them all at 3/70 and accompanied by my Frosty+19. They should be a fun team. Minions galore.
King Arthur - I don’t have him. I think he is good against red titans. I have Frida though. I don’t know which one is better overall.
Kingston - I don’t have him. Wanted to. But the boat has long sailed ago. Maxed 2 Liannas instead since I have her costume.
Lianna - I have 2 both maxed on the original and costume version. One at +7 original version.
Morgan - Currently leveling mine. Original plan is to get her to 3/70. Both she and 3/70 Ratatoskr will be competing for my 13 tonics, but initially plan to only ascend one of them.
c-Melendor - I have 2. Same thing what I did with Sonya and her costume.
c-Caedmen - I have 2. Same thing what I did with Sonya and her costume.
Mother North - Just recently maxed mine obtained early this month. No emblems yet but she is sturdy and serves as my safetynet on my second monogreen team for wars. Also used her on the right wing this last very fast war (on the left wing was Albe+19; tank was Telly) and I was never one-shotted.
Bertila - This beauty still has eluded me. Might not be getting her. My last hurrah will be on the Valhalla portal where I have 500 coins. I only love her for her art, nothing more.
Brynhild - I had two copies. After maxing the first, I fed the second for me to free up roster space. The maxed hero only saw action once. Never again. People say she is good. Maybe. But not for me.
c-Boldtusk - I maxed 2 including their costumes. Both are emblemed on the original version since I need that attack buff more.
c-Marjana - I don’t have her costume, but the original version came from my Training Camp and placed her at 3/70, awaiting for costumes. Once obtained, I may max 2 of her and the costume. 29 rings in my inventory.
Khagan - This hero is one of my staples to my Hero Academy level 10. I have leveled one to the third tier of ascension (originally planning to get him to 3/70) but was bumped down because I recently got Kestrel. He will be leveled further after Kestrel. I also have his costume. I will have to let somebody else max their Khagan and listen to their feedback before I use some of my hoarded rings.
Just my 2 cents.