Options (Boosting): Little Sage, Marron, Wingal, Starlight Unicorn, Lake Maiden, Lien, Knight of Rose, Morganna, Protector of the Airspace Knight Gryphon, Knight Squire, Alen, Lionmane Stallion, Mirubiru
Options (Attacking): Blaster Blade, Covenant Knight, Randolf, Knight of Silence, Gallatin, Knight of Conviction, Bors
Backing all winning images are a line of strong rearguards. Your vanguard is irrelevant if your rearguard can't support it, and vice versa; there are two types of rearguards dedicated to this purpose, and creating a strong balance between them is the backbone of any good deck. These are the boosters and mainline attackers, which with several small exceptions, consist primarily of grade 1 and 2 units.
These units are best tackled together, as they are typically packaged within the same set together so that their intended formation is obvious. To begin with, the Little Sage, Marron represents a type of universal booster that every clan possesses; a grade 1 with 8000 power and no skills. Mathematically, any given deck will only need a booster with 7000 power(because all relevant levels of power are divisible by 5000 and most grade 2s have at least 8000 power) but Marron and his many counterparts are not a bad foundation for boosting.
The first instance we get of a "strong" booster is Wingal, who has the misfortune of having a very unintuitive skill. In theory, his ability to grant +10000 total power to a Blaster Blade unit is great, but when you examine this skill more closely, Marron already handles the relevant boosting not just for Blade, but for any unit you place in front of him. Remember that nonzero shield scores are dealt with in units divisible by 5000--Blaster Blade can intercept with a shield score of 5000, Epona can guard with a shield score of 10000, and so on. What this means is that to get past the opponent's defenses, you need to think in units of 5000(i.e. 5000 --> 10000 --> 15000 --> 20000 --> 25000.) So when Wingal boosts Blade, their total combined power is 19000, a score which is functionally the same as 15000. Because Wingal has just 6000 power as a rearguard, instead of boosting 8000-power units up to 15000 he's boosting them to 14, just small enough for the enemy to stop with a draw trigger.
The solution to this is Starlight Unicorn. When called, Starlight Unicorn contributes +2000 power to any Royal Paladin vanguard or rearguard unit. By giving this power to Blaster Blade, his combined strength with Wingal is 21000, enough to make the opponent waste between two and four cards. This is a significant hit to their hand, especially if you could handle it on multiple rows.
Unfortunately, this just isn't that great of a setup. You have to call three different units to make it happen and it only lasts for one turn, where most other Dozers can work continuously for the entire game and only take two cards from your hand.
Lake Maiden, Lien is the ideal card to fill the gap that Wingal would leave. At 7000 power, she can bring any grade 2 or 3 of at least 8000 power up to 15000, and in a secluded list of special cases, provide the support necessary to reach the 20000 absolute borderline. Furthermore, Lien possesses a card changing skill that lets her drop one card to draw one card--while not as nice as paying for the cost of calling her, this helps search the deck for cards with needed skills, or to discard certain less useful units in order to gain a shield. This also makes her ideal for riding when taking the first turn, as Lien can ensure that this turn is not put to waste.
Much to my dismay, the Knight of Rose, Morganna also falls in with Wingal's crowd. By dropping 1 Royal Paladin card from the hand, she will gain +4000 power, but only when attacking. This is a deliberate destruction of the hand, which in Vanguard is inadvisable on the best of days. The hand's direct importance to the game is the reason that the Nubatama clan has so few cards in it, because hand destruction is very dangerous and the deliberate elimination of it by a unit that does not at least possess intercept or a viable way to reach the absolute borderline is suicide. Her low power also eliminates much of her usefulness as a booster.
Protector of the Airspace Knight Gryphon is on Morganna's heels. By soulblasting 1, Knight Gryphon can gain +3000 power, but only when attacking. As only grade 2 units can intercept, and most Royal Paladin decks would prefer to save their soul for either Soul Saver Dragon or a megablast skill, Gryphon is relegated to a similar level of obscurity.
Making up for these is Knight Squire, Alen. His skill is a stackable counterblast 1 which gives him +1000 power, allowing him with a CB2 to pass cards like Randolf or Baron through the absolute borderline. While less reliable than some cards to come, Alen should be considered and weighed carefully with Lien's draw power--a number of early decks may even be able to run both of them together.
Following Alen's train of thought but in Wingal's spirit, Lionmane Stallion is a 4000-power unit which soulblasts 1 to give an "Alfred" unit +6000 additional power. Observant viewers might notice that with a vanguard King of Knights, this skill is largely useless. However, Stallion ultimately brings out the hidden potential in a deck which runs four of both the King of Knights and Alfred Early, as this puts them both into the absolute borderline. This card was in fact so good that, four sets after its release as a promo card, it was re-envisioned as the Blaster-oriented Apocalypse Bat.
Like Aermo before him, Mirubiru is an offensive version of Lien, oriented toward boosting. Perhaps if he shared her level of power, then this card would be worth the risk of use, but since his skill is a simple card change instead of an actual addition to the hand, there is not much reason for the opponent to guard him. Due to the aforementioned low power, I cannot recommend his use.
In terms of attacking units, the Covenant Knight, Randolf is a unit that has arrived before his time. While he can rise from 8000 up to 11000 before boosting, this requires that one's hand size exceed the opponents, which is simply not a factor that the player can control. In addition to this, there is no boosting unit with a power of 9000 that can support Randolf in reaching the absolute borderline.
Because of this, it is more practical to run Royal Paladin's vanilla alternative, the Knight of Silence, Gallatin. At a static 10000 power, Gallatin functionally fulfills Randolf's role better than he can.
There is however, one unit which outshines both of these. Packaged in Trial Deck 01, the Knight of Conviction, Bors achieves on his own what both Randolf and Gallatin fail at. For a relatively low cost of counterblast 1, Bors will gain +3000 power; while the skill does not stack, it can be repeatedly easily, and when combined with a 7-8000 power unit like Lien or Marron, this will immediately pass through the absolute borderline. While most Royal Paladin decks prefer to save their damage for more costly counterblasts, Bors is one of the most immediately useful grade 3 units of all because of how readily he wastes the opponent's hand.
As a closing note for this booster pack, please keep in mind that despite my earlier comments regarding Marron, he does have his place as the primary booster for most vanguards, as they tend to clear 12000 power individually.