There are various types of riverseeds, but Blue is a Sylvan, which brings nature to mind. Riverseeds are a valuable treasure. It’s fortunate that Heaven’s Glory School had one for Lindon to steal. ;)
Eithan Arelius reflects on the nature of Riverseeds in Blackflame, book 3, relevant section pasted here:
Sylvan Riverseeds were natural spirits—beings like Remnants, only born of accumulated vital aura rather than the death of a sacred artist. They only formed in places where the aura was both extremely strong and in perfect balance. . . If the aura slanted toward one aspect or another, a different natural spirit would form. Typically, you would find that balance of aura in the heart of a forest, next to a spring or a river. In such a place, air and earth, heat and cold, life and death all coexisted at the same point in roughly equal amounts. . .
Other Sylvans were better suited for different purposes, but Riverseeds were gentle and flexible. They could work with power of virtually any aspect, supplementing and supporting other forces. Which made them excellent raw materials. They were so malleable that a skilled craftsman could make a Riverseed into a guardian, a weapon, a guide, an elixir, a power source, a drudge, or—in some cultures—a very expensive cocktail. (Blackflame, ch 10)
Born only a vaguely defined blue spirit seed with the capacity to develop into any form, Little Blue becomes female, probably because that’s how Lindon perceived her. When we first meet this character, she is only the size of Lindon’s little finger, blue and vaguely human-shaped. For various reasons she becomes more defined over the series, eventually developing arms, legs, fingers, toes, and becoming more capable in mind and body. At optimum strength she is deep blue in color, fading to robin’s egg blue when weakened and/or hungry. In Wintersteel Little Blue contracts with Lindon allowing her to grow to about a foot in height and making her body even more detailed, and a lighter shade of blue.
Generally cheerful, playful, helpful, and friendly. Compassionate and caring. Brave. Loyal. Stubborn. She believes in Lindon, and can read his spirit to large extent. Fun-loving. She likes to be with people, to play, to cuddle, and to sleep near Suriel’s marble.
At first, she shied away from Yerin because of her blood shadow, but eventually came to love her as well.
Yerin darted over to the Riverseed, scooping her up in her bare hands. The spirit squirmed out of her grip, scuttling over to hide behind Lindon’s leg. She bared her teeth at Yerin in a threatening grimace. Yerin’s face fell. “She doesn’t like me?” Lindon was as surprised as she was. (Blackflame, ch 14)
Little Blue has a compassionate nature with the heart of a healer. As a pure Sylvan Riverseed, she has the power to cleanse spirits and soothe madra channels. She frequently helps Lindon and Orthos cleanse their spirits of Blackflame madra. In book 3, Blackflame, she helps Lindon mend the broken channels in Jai Chen, thus (unintentionally) helping to untangle and free her dragon-spirit, Fingerling.
Lindon set Little Blue onto the Emperor's shoulder. She leaned into his neck, pushing a bigger orb of blue into his madra channels than she had used on Yerin. It took her a hair longer this time, either because she found it harder to affect an Overlord or because the field was affecting her more strongly. (Underlord, ch 16)
Little Blue eventually learns to fight, as foreshadowed in book 3, when Eithan Arelius reflected on the many possible applications of a Slyvan Riverseed:
Riverseeds were gentle and flexible. They could work with power of virtually any aspect, supplementing and supporting other forces. Which made them excellent raw materials. They were so malleable that a skilled craftsman could make a Riverseed into a guardian, a weapon, a guide, an elixir, a power source, a drudge, or — in some cultures — a very expensive cocktail. (Blackflame, ch 10)
Little Blue, who bonded with Lindon in Wintersteel, has received some measure of ability with Lindon's pure Path. By book 10, she can create the Empty Palm — large enough to interact with humans. Also, she can use a little bit of the Soul Cloak, and she can partially manifest the Hollow Domain. She demonstrates these powers when sparring with Wei Shi Kelsa and Jai Chen. She also spars with young members of the Twin Star Sect and with Jai Chen’s tiny dragon-spirit, Fingerling.
Little Blue likes to be social. Even in sleep, she like to cuddle Suriel’s blue marble, and when awake she rides on Lindon’s shoulder. She doesn’t like to be safely hidden away in the void closet. In book 5, Ghostwater, she swims and plays with other tiny spirits in Northstrider’s well. She enjoys playing with the little toys Lindon puts out for her in Uncrowned. In book 8, Wintersteel, Blue loves to zoom around on her own tiny cloudship. In Bloodline, she loves swimming and diving in her own pool, on the cloudship Windfall. She likes play-fighting with Fingerling, and riding around on tiny-Orthos in book 10, Reaper.
For those who listen carefully, Blue does communicate. Over the months she gradually develops the ability to communicate through chirps, coos, bell-tones, and whistles. Dross quickly learns to decipher her speech patterns. Akura Mercy seems to understand her at times. Because of their bond, Lindon can directly feel the emotions behind Little Blue's words.
“What's your name?” Fury asked gently, and Little Blue gave a series of chimes in response. “That does make sense. Are you having fun here? No problems? ...yeah, training can be lonely. You'll be back with your friends before you know it. Speaking of friends...” He turned to look at Lindon, and so did Little Blue. “What do you think of him?” Lindon had never before been so worried about Little Blue's opinion. She made a long, complicated whistle. It carried more meaning than Lindon could untangle. Fury's drifting, shadowy hair rose to a point. “Really? Well, I look forward to that day.” Little Blue squeaked and hopped off his shoulder, sliding down his robes like a child down a hill. Then she scurried back over to her toys: a pile of miniature junk that Lindon had arranged for her to play with. (Uncrowned, ch 4)