I like the fluff and lore and concept of this class. However, I have to agree with the last poster, I feel it may be a little to strong. No one ability is necessarily OP, but all of them are quite useful and with the host of them it just seems a bit to much. And so I'm going to over analyze it to death, for the record I only nitpick on classes I actually like, so don't take this as criticism but complement :)

I look at it and this class fills three 'roles'

1. Standard melee damage, competent at this, nothing too special unless you focus your subclass on it.
2. Utility, protecting allies and boosting their healing; pretty good at this.
3. Big survival boost to surviving for long lengths of times.

I think that 3 is the problem. It gets an extra 2d12 plus 4 constitution to boost it's health pool. It can heal whole 3 die of damage once a *short* rest, this one feels pretty strong and I may move to long rest. It gets a not-trivial amount of healing every time it damages someone, and if all that wasn't enough it gets a the boost from healing spells, quite powerful post-battle, and doesn't even die easily when reduced to 0 hp. It even gets some abilities to boost it's saving throws to avoid indirect death-by-magic. This class just won't die. Which is cool, it's not an issue thematically for a class to have. However, this survival and competence at melee (if sub-classed for it) would be almost enough to justify a full class in itself without the extra utility the class also gains.

*yes I know I listed a number of self-enhancements in this list, so a class could choose not to pick them, but imagine a class that went for survival sustain and this argument still applies, the fact that they get even more flexibility to specialize beyond that only makes the class more potentially powerful.

I realize the survival exists to support the utility, to make full use of the class utility your have to give up personal survival to heal others (though even without using blood pact reactions it still has a decent bit of utility). However, the problem is that you get a choice, you can use your survivability with blood pact to provide good utility, or you can use your survivability to be a strong melee class or tanking class, and at no point do you have to sacrifice one role to do the other. One fight he can tank, another he can give his party sustain, he can switch between the two roles at will and that's pretty flexible for a class that felt nearly complete without it's utility. Put another way, the survival works equally well in a one V one fight as it does in tanking as it does with utility, it's a utility class that didn't have to sacrifice much personal power to be a utility class.

I can't put my finger on just how strong the class is, I don't think it's utterly broken or anything, but I suggest considering separating out some of the party utility and the not-dying a bit more. I know that sounds odd, since much of the utility is built around the not-dying traits a little more, so one has to choose which to focus on. I know this seems odd, since the survival exists only to support the utility, but I think party-sustain utility can be built into the class that does not substantially boost the classes one V one or tanking survival to better force a choice between personal survival and party utility sustain.

Assuming you agree with me at all and want to do something I could see two options. Either remove some of the core abilities, or just give the class weaker core versions, and move the power of those abilities more into the four existing 'sub-classes', or if deciding how to divvy abilities up between subclasses is too hard you could go with a separate sub-class choice at higher levels which chooses between utility and self-sustain and/or force knights to pick between one of two affects at a few levels to force them to make self-sustain or utility choices as they level. The affect is the same, move core abilities more into a choice between these two options.

The "bloody survival" sustain kit could get more of the effects that boost available HP and sustain. I'm thinking this may be the only version that gets crimson revelry heal, or make the heal even weaker by default and let this focus boost it current or slightly better levels. However, it would trade most of the utility affects that come with blood pact, and likely will have a rather limited use per day for blood pact reaction. This build choice focuses on solo power, or perhaps tanking, but trades off much of the utility.

The "blood is thicker then water" or whatever you call it utility focus obviously could get more of the utility abilities linked with bonded allies. However, the big thing I could see for boosting utility and keeping with the fluff is to replace self sustain options like crimson revelry with party-sustain-through-bloodPact. As it is now once you get the ability to share your half competence bonus to heals affect with allies there is no reason to 'waste' a reaction on bloodpact, unless someone would die if you didn't use it or your at 100% max health and would waste the crimson revelry self heal, if your tanking damage in the frontline anyways your never need to use blood pact to take damage which feels odd for your core class ability.

To address this I'm thinking the "utility" role may gain the ability to decrease damage dealt to the knight when he uses blood pact by an amount based off of your constitution modifier, such that every use of blood pact mitigates a small amount of the total damage dealt to the party (I like this option anyways, fluff wise and utility wise, so consider making it one of the self enhancement options at the least please!). This still fits the 'feel' of the class at mitigating damage with the power of BLOOD, but by tying it directly to blood pact use it makes the ability only useful for mitigating party damage, not for making the knight a unkillable machine in 1v1 or even if he is playing tank; ie makes it only utility at expense of personal power. A list of other 'utility' options, which all feel like they could be made potential self enhancement feats, or could be incorporated into a more utility focused knight by removing some of it's survival and self sustain options are below.

1. After using a reaction to take damage for an ally a passive effect is left on the ally until your next turn This could be:
a. allowing a small amount of damage to flow to you each time the ally takes damage after the first reaction to protect them, to boost the amount of damage that can be mitigated with a reaction. This risks slowing the game down if you add too much math per hit, so probably this should be made something very easy to calculate, like you taking a flat X hp of damage that otherwise would have been absorbed by an ally per extra damaging hit the ally takes.
b. Provide the ally some small bonus to future attacks taken between your reaction and your turn. Maybe advantage on next attack the ally takes if before your turn, but not sure advantage feels right. I could see something like the ally gaining temporary hit point equal to your constitution modifier (give or take some extra amount depending on rather this is built into utility route or requires sacrificing an enhancement to get) which lasts until the ally turn or the next time you use a reaction to protect an ally, whichever comes first.

2. As a reaction give an ally advantage on next attack role against the, BUT you take disadvantage on next role against you, this is low power ability suitable to role right into a utility role designed for a non-tanking knight, it's situational since most knights will expect to usually get attacked and thus suffer their disadvantage. if this is an self-enhancing feat boost it by giving ally bonus on next two damage roles in exchange for your taking disadvantage? or perhaps you can give advantage to an ally in exchange for disadvantage on next defense role made before your next turn, such that smart planning can give advantage to ally at a time when it's unlikely that your get attacked before your next turn to suffer disadvantage, but limit number of uses of this ability

3. Allow any heal on yourself to be shared with an ally. This in fact could be done in a number of ways
A. any heal that increases your HP above 100% is shared with all other allies evenly, but the professioncy bonus increase to healing only happens once even if you have the ability to share this with bound allies.
B. Share exactly half of a heal with a blood pact ally of your choice, possible using reaction or X times per day to power this.
C. X times per day you can have your Crimson Revelry heal be given fully to an ally instead of yourself, if this is an enhancement possibly give some tiny boost to the heal shared this way, or alternatively replace it with a very small number per day to sacrifice your crimson heal a turn to give it to every other bound ally.

Finally, the Crimson Revelry and Natural Healing feats synergize in a way you may not have intended. Do you intend for both affects to activate to gain constitution modifier + 1/2 profession bonus each heal? I would specify explicitly rather or not they 'stack' in your description.