Sakuya has a strong build, excelling at both combat and investigation, able to contribute to social combat if not take the lead. However she's still a starting character and has vast room to grow. Here's some suggestions for how she might be advanced with experience, which should give insight in how to plan your own characters' advancement.
You can rely on gaining 4 experience points per session of play. So if a Charm costs you 8XP and you play weekly, that's two weeks of real-time.
Getting to Essence 3 is a priority for all but the shortest games: it's only 16 XP, gives you access to more powerful Charms, and 10 extra motes of essence. Best of all, it has no training time. Essence ratings of 4 and above require extensive downtime and more XP - grab them if you can, but don't rely on getting the opportunity unless you're in a long campaign.
Charms are your character's most important source of growth. They're powerful, have a flat cost, and most importantly provide you with a new capability. Raising Athletics from 4 to 5 for two sessions' XP isn't very exciting - you already know how to use Athletics, you just roll an extra die now. Buying Mountain-Crossing Leap Technique for roughly the same XP gets you thinking about what problems you can solve by jumping 20 miles at a time.
Sakuya is Dawn caste, and her primary responsibility to her Circle would be winning fights. However, it's more fun for everyone if she can contribute outside of combat, and her Ability spread is perfectly set up to do so. An Awareness Excellency and the various Keen (Sense) Technique Charms would help exploit her talent for perception. From Investigation, Judge's Ear Technique, Courtier's Eye Technique and Know the Soul's Price greatly enhance Sakuya's ability to gain information about people, beefing up her value as a detective and in social combat. From Integrity, an Excellency and Elusive Dream Defence would significantly bolster her defences in social combat, which could be vital in the face of Unnatural Mental Influence. EDD's prerequisite, Temptation-Resisting Stance relies on Temperance and isn't much use to Sakuya unless she can create a custom version or stunt it to apply to Valor or Conviction.
Athletics Charms are an excellent investment, as they have applications in and out of combat. An Athletics Excellency would have a wide variety of uses, and allow the purchase of Thunderbolt Attack Prana, a powerful damage-enhancing Charm. Soaring Crane Leap, Lightning Speed and (from Glories of the Most High) Racing the Sun are options for increasing Sakuya's movement speed. Glorious Temple Body (from the Ink Monkeys blog) provides passive bonuses to feats of strength, movement and jumping, while Increasing Strength Exercise grants a persistent increase to Strength in return for a hefty commitment cost.
In combat, Sakuya is best served by increased investment in Melee Charms. Fire and Stones Strike adds post-soak damge dice, very useful given the low base damage of Sakuya's daiklave (and is upgraded further by Shining Razor Wind from Glories of the Most High). Sun Sword Concentration and Molten Sun Blade (both from the Ink Monkeys blog) help against soak and hardness, perfect for a messy breakup with Misha. The Extra Action tree, starting with One Weapon, Two Blows allows Sakuya to pressure opponents with DVs too high for normal flurries. Solar Counterattack and Ready in Eight Directions Stance can deter or kill an opponent trying to melee flurry. The various Charms in the Call the Blade tree protect against disarms, and ultimately allow ranged attacks with the Melee Ability. The Solar Melee tree is one of the best in the game; nearly every Charm is a great addition to Sakuya's panoply. In general though, plunging to the end of the sub-trees one by one will get better results than splitting your attention between them.
Infinite Melee Mastery is a priority target. Each Ability has such a Charm, allowing the commitment of motes to gain a discount on Excellency activation; as Sakuya uses her Melee Ability dozens of times in the average fight, that discount allows for vastly more bonus dice than she could usually afford.
Finally, Panoptic Fusion Discipline is an Awareness Charm from the Ink Monkeys blog that grants a persistent Aim bonus for a scene without needing to take an action. This is a priority Charm for Sakuya.
In general, raising Attributes and Abilities in play is a bad idea. They have a flat cost at character creation, but their XP cost is a multiple of their current rating. It's more efficient to max out the most important traits before play, saving XP for Charms; Sakuya is obviously built on this principle.
However, it's not possible to anticipate everything you'll want or need at character creation. The events of play could make any sort of purchase appropriate (it's easy to see Sakuya making an effort to be a nicer person and improve her Charisma, for example). Thankfully while building a strength by improving a mediocre trait is expensive, covering a weakness is cheap. Raising an Ability from 0-3 costs the same as a Charm, and tends to make a good story as the character struggles from nothing to competence.
It's not worth buying up infrequently used abilities as insurance - stunts and Virtue activations can usually get you over the line on the rare occaisions where you can't rely on your Circle to take care of it. It's best to concentrate on Abilities that will allow a concrete benefit by giving access to some low-hanging fruit in the Charm trees, or that have proved critical to the campaign's focus. For example, Sakuya would definitely benefit from raising her Resistance enough to qualify for Spirit Strengthens the Skin and Iron Skin Concentration as a backup for when her parry DV fails her. If she found herself in a campaign where interaction with spirits was frequent and critical, raising Occult would be valuable no matter how thoroughly her Circlemates had it covered. Likewise if the clash of armies became a focus, she'd need to invest in War to operate at full capacity on the battlefied.
The one Ability purchase almost certain to be useful would be an extra dot of Linguistics to pick up the Old Realm language. Remember the death poem Sakuya couldn't read? Not only was that the language of the First Age, it's still the native tongue of gods and demons. An extra Athletics specialty in Leaping would also be a guaranteed winner.
On the Attribute front, Charisma is an excellent candidate for an upgrade, since Sakuya's ability to make Investigation based social attacks is undercut by her poor attack Attributes. An extra dot would be 4xp, two would be 12 (and I think Charisma suits her better than Manipulation). Wits isn't bad either, as it's used for Join Battle and often substitutes for Perception in time-constrained situations, but the 8XP used to raise it could be a Charm instead!
Sakuya can't raise her Willpower, Valor or Conviction - they're cheap in character creation and expensive with XP, so naturally they're already at max. Raising Temperance would be cheap, but the benefits would be modest and buying two dots in it would subject her to a fourth set of Virtue compulsions. Best to leave this area of the character sheet alone.
If your group requires you to pay XP to gain backgrounds, that's like taxes - you just have to put up with it. If you have the option, it's more like insurance, and an "easy-come, easy-go" attitude will keep your XP free for Charms (unless you're emotionally attached to a particular Ally, Artifact etc).
Either way, Sakuya should aim at getting a Manse for its Hearthstone. She'd gain a bonus power, enhanced essence regeneration, and fill the empty socket on her daiklave.
She should certainly get better armour. As she doesn't rely on her Dodge DV, a mobility penalty would only impact her leaping and running distances, which are prodigious and can take a hit. 17/17 soak and 11/11 hardness from orichalcum superheavy plate would absolutely help the cause.
More weapon options might also be in order. A Grand Daiklave would sacrifice Speed, Accuracy and Rate in return for a massive Damage bonus (including Overwhelming 4) - and if you've picked up Iron Whirlwind Attack you won't care about Rate. This would be useful against high soak, low DV targets like giant monsters and many Dragon Blooded, so having one available would be worthwhile.
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