Chibi

Chibi (チビ, Chibi) is the Priest of Renji's party, Chibi isn't her actual name it's just what everyone called her because of her small stature, and she adopted it.

Personality
Chibi is a quiet girl, she barely ever talk and doesn't interact much with almost everyone else. The cause of that quietness probably being shyness as even when she was force to by the situation (a fight) she had trouble speaking to Haruhiro and barely talked to Yume during their boat trip despite them being together for quite a time.

However, Yume notes that despite her weirdness, she is a sincere and earnest girl who would do anything for her comrades. She also notices that, despite how it may look, she helps her team in many unremarkable ways and is way more cautious and aware of her environment than most.

From the short story where she is the main character, it seems like she also suffers from low self-esteem: as she believed that her life had no value before she met Renji.

Finally, and despite all those traits, she is surprisingly expressive as shown by how she blush when interacting with Renji.

Appearance
Unsurprisingly, Chibi's most notable characteristic is her height as she is below 150 cm, her voice match her size and is just as tiny. Most people describe her as "adorable" and "mascot-like", however Haruhiro notes that when she appears alongside Team Renji even her start to look somewhat threatening "like a mascot with some unknown and threatening powers hidden within her".

She is never describe more than that in the text, but both the light novel's images and the anime show her with a round faces, short green hair, and two pigtails on top. However, in the LN her hair are a more vivid green than in the anime, the pigtail are made using ribbon with sleigh bells, and her eyes are grayish instead of being the same dark green almost black used in the anime.

While similar to other priests uniform, hers is of better quality than at least Manato's and Merry's, using high quality fabric with decorative touches on the edges.

Since, like most character in the series, her age remains unknown: we don't know if she is a really small adult/teenager or an average size young girl. Although, the fact that she doesn't seem to have grown up since the star of the series, make the former more likely.

Level. 1
She was part of the group who arrived in Grimgar at the same time as Team Haruhiro and was part of the party put together by Renji.

Level. 2
She helped fight back the orc attack on Alterna by defending her party back line and healing Renji at the end of his fight against Dogran.

Level. 3
She was part of the joint expedition with the regular army to take the 2 orcs fortresses near Alterna. She, along the rest of her team is part of the team attacking Dead Head Keep, moreover she played a part in the fight against Zoran Zeshuu and his retinue even being among the one to pummeled him by the end of the fight.

Before Level. 14++
We don't precisely know what she did but as a member of Team Renji she traveled to the Red Continent which they explore for the time, only coming back to Grimgar after the death of Sassa.

Level. 14++
Yume meets her on the boat to Grimgar as Team Renji was coming back from the Red Continent. As usual, while on board she barely talked to anyone but did somewhat interact with Yume as she was the one healing her after her training session with Momohina. Moreover, Yume notes that contrary to what it may look, Chibi actually help her team quite a lot, supporting them in a lot way which don't stand out.

Like most of her team, Chibi didn't suffer from sea sickness.

Before Level. 17
Since she was in Alterna at the time, she took part in the battle against the Southern Expedition and while the volunteer soldier corpse did lose that battle she, along many of them, managed to escape and began to attack enemy positions in the frontier.

Level. 17
She was part of the Volunteer Soldier assault on Mount Grief and part of both the fight against the Lich King in the Graveyard bellow and the Orc in the Castle above.

Abilities
Chibi is an experienced healer able to skillfully heal her allies, she is also able to use offensive light magic, like Judgement.

She also is a skill fighter able to beat even an orc using only her priest staff, as such she takes on a defensive role and protect the rest of her team's back line while the frontliners are fighting. Moreover, while she isn't the only fighting priest, she is remarkable in that, unlike someone like (for example) Tada, she has only ever been a priest and is of very small stature.

However, she is pretty lacking when it comes to interpersonal skills and has trouble communicating with other.

Finally, while it hasn't really been an advantage yet, the fact that most people tend to underestimate her at first, base on her cute appearance and demeanor, could play in her favor since unlike a genuine mascot like Anna-san she is more than able to fight back.

Renji
While their personality make it hard to get a really clear read on their relationship, it's obvious to see that they have a lot of affection for each other. However, the nature of that affection does seem to differ.

On one hand, Renji treats her like a younger sibling: congratulating and patting her when she does well while also recognizing that she is an excellent team member despite her small stature.

On the other hand, Chibi's feeling for Renji are way stronger, the deep blush she displays when he pats her already shows that she likely loves him. However, her actual feeling toward him seem closer to worship, her PoV in Level. 4's short story "Chibi-chan Cooking" showing that she basically lives for him.

Trivia

 * She doesn't say the whole chant when casting spells.
 * We don't know her true name, Chibi does but doesn't use it, and it's unknown if she has told it to anyone.
 * Her birthday is on April 2nd (source, unknown).
 * If her current height is indeed her final height, she could be considered to be a dwarf (the medical type) as we know that she is smaller than 150 cm and the usual threshold for dwarfism is 147 cm.