So some math on Echo of Peace:
Team GEAR
James Robert
With a Base Resilience of 500 and his ability 'Merely' Human James can only take damage, above the 25pts of strain damage caused by 'Merely' Human, between the ranges of 1,429 and 1,711. Which I suppose technically widens the threshold at which damage occurs, from 501 to 599 however I think James wouldn't really mind that since it means he can no-sell more damage then most Magical Girls.
Michael Rodriguez
With a Base Resilience of 500 and his 'Atheist' ability Michael isn't quite as tanky as James however he is still quite resilient, especially with regards to magic based damage. All damage less then 1,428 is negated and he has an effective Health of 7,141, which goes up to 14,282 for magic based damage. Plus depending upon if Atheist is applied before or after Base Resilience he might be able to no-sell up to 2,856 magic based damage.
Samael Balier
Samael is quite interesting here. We don't know what exactly Empower and Shell do so it's hard to factor them in. However I'm going to assume that Empower is offensive only, it fit's the name, while Shell works on defense as well as offensive, it has Defender as an ability after all.
So that means by default Samael can negate 500 + 1d25 Magic Modifier + 1d1000 Shell damage which gives a range of between 502 and 1,525 with an average of 1,013. Applying Echo of Peace to that ups the range to between 1,434 and 4,357 with an average of 2,894. That said his effective Health is much easier since it's just 7,141 regardless of circumstances.
Angel Gabriel
With a Base Resilience of 1,000, a Magic Modifier of 400, and her Armor Of Light ability Gabriel is really tanky. Not quite much as James but she's still pretty tough.
With Echo of Peace she can no-sell between 2,860 and 4,000 damage with an average of 3,428. Her Effective Health is a quite astonishing 35,712 which puts her pretty high up there.
Other Noteworthy Allies:
Magical Girl Goddess Gold
Obviously Rei is total bullshit as expected. First up there is her 75% chance of no-selling any attack that lacks the Divinity Affinity. Next is her Base Resilience of 1,000 +1d1000 Magic Modifier for a no-sell range between 1,001 and 2,000 and average 1,500 which Echo of Peace effectively raise to between 2,860 and 5,714 averaging out at 4,285. After that is a flat 75% damage reduction, much like the one Nepgear has, which stacks with Echo of Peace to give Rei a stupidly high effective Health of 628,561.
No one will be killing Rei on Nepgear's watch!
Magical Girl Truth
Hilariously enough she's like fine china compared to our Team GEAR soldiers. Her Base Resilience of just 125 and Magic Modifier of 50 mean she has a no-sell range of between just 360 and 500 averaging 428. So even with Echo of Peace Nepgear is still harder to hurt (568-737 Avg 652).
Her Effective Health is a bit better at 7141 compared to Nepgear's 3,600 in fact it's even on pair with the mundane troopers in Team GEAR.
Magical Girl Carnage
Her Base Resilience of 500 and Magic Modifier of 1d100 gives Carnage a no-sell range of between 1,431 and 1,714 averaging out to 1,571. Her Effective Health comes in at a nice 8,569. That said as long as she is Intercepting an attack she's immune to damage which does further boost her durability
Counter Force: True Anti-Demon Tank
What? It's on the list of known Allies so it should benefit from Echo of Peace. Incidentally tanks become super tanky when given a magical defense, who'd a thought? The tank flatout no-sells any damage less then 5,714 under Echo of Peace and can endure an impressive amount of damage with an Effective Health of 71,426.
I foresee "Nepgear: Tank Commander" as a potential future. I wonder if Core Connection would work on tanks? I'm sure Nepgear would object less to kissing them then people.
Magical Girl White Knight
With her Base Resilience of 750, Magic Modifier of 1d1000, and Might of the King ability White Knight is fairly tanky. They give her a no-sell range of between 8,571 and 11,425 averaging out at 9,997. Her effective health is 12,855.
So it's certainly an impressive ability and I certainly think we should be pretty high up on our priority list. However we just spent a great deal of xp on buffing our teammates and they are almost certainly going to get Power Armor soon, this is the third time they've faced ultra high level foes so they are actually quite good for testing our the full capabilities of Power Armor, for even more enhancements. Nepgear meanwhile is falling a bit behind.
As it currently stands her teammates are better at basically everything besides tanking damage and flying. Even then both James and Gabriel at better at tanking, with the others are around the same level as her, and Gabriel's flight is twice as fast, if not as maneuverable, as Nepgear.
So for the time being I absolutely suggest focusing on improving Nepgear rather then buying more team/ally buffs. Of the personal upgrades I'd probably rank Indestructible Core as the most important because it should make keeping Nepgear alive easier and let her fulfill the traditional role of a Magical Girl, tank to her allies DPS, while we get more xp to round her out.