Concept: As the paladins’ relationships with the Lions deepen, the Lions become able to lend them a hand in on-foot combat via “symbiote form”.
For the five earth paladins, they can only do symbiosis with their “archetypal” Lion- the first one they bonded with. Shiro to Black, Keith to Red, Lance to Blue, Hunk to Yellow, Pidge to Green. Active symbiosis is indicated when the affected person’s irises and the veins on their arms and legs start glowing in the lion’s color. They also gain tattoo-like marks on their face based on the shape of the lion’s crest.
(It should be noted it’s only the iris that lights up in the eye- the lion’s color never covers the pupil or extends into the sclera)
In this state, the paladin’s body is reinforced and their physical traits are bolstered similar to the Lion’s capabilities. For example, all paladins would get faster in their symbiote form, but Keith would be noticeably faster than the other paladins. Durability increases, but for Hunk he’s virtually indestructible. They can also wield the Lion’s elements to a degree- Pidge generating vines, Lance spraying ice, Keith shooting fire, Shiro with some limited flying abilities, Hunk able to create concussive shockwaves.
Allura can do it too, but in her case, it works a little differently- her symbiote forms are weaker (not as much of a boost from her default) but once she establishes a connection with one of the Lions, she can use their symbiote form, allowing her to use all five Lion symbiotes provided she puts in the work to establish those connections first.
It has drawbacks- overusing it can tire the paladin out and if they push it really hard, they basically enter “burnout” and can’t use symbiote form until they recover (on top of being exhausted and sore), it’s susceptible to anything that weakens the paladins’ connection with the Lion.
The power that you can channel through a normal body without absolutely frying it is much less than the Lion itself is capable of, so a paladin in symbiote mode couldn’t fight a robeast like a Lion could- because the Lions understand that their buddies are squishy and made of meat and aren’t going to push them to dangerous limits.
Which is another drawback- while it’s rare, Lions can disagree with their paladins about whether it’s fine to keep pushing ahead and using symbiote form, and cancel it on them. One party can’t hold symbiote form against the other’s will so if either of them back out, it stops right away.
It also doesn’t heal any wounds the paladins sustained beforehand, but gives them a sort of adrenaline boost-like effect to power through it… temporarily. Once they revert, it’s all waiting for them right where they left it.