Tsukishima is very tall and has an incredibly slim build for a volleyball player. Despite being a first-year, he is the tallest player in Karasuno High, standing at 190.1 cm. He has short blond hair, pale skin, thin eyebrows, and golden-brown eyes. He is often seen with a frown or a condescending smirk on his face.
After joining the Karasuno volleyball team, Tsukishima wears the standard Karasuno volleyball uniform (black, orange, and white uniform with the number 11 on the back) during matches. He also wears knee pads and white volleyball shoes with green details. He is also the only player on the Karasuno team who wears glasses. Since Chapter 109, Tsukishima began wearing prescription sports glasses (a gift from his older brother, Akiteru) in matches.
Outside of matches and practice, Tsukishima wears a black gakuran, the Karasuno male uniform. He is usually seen with white headphones around his neck. The brand of these headphones seems to be "Somy", which is a play on the real name brand "Sony." He has been seen wearing various shirts with crescent moons on them, which plays on his name "Tsukishima" with "tsuki" meaning "moon".
Tsukishima is very blunt and will sometimes rile his teammates up due to his antagonistic nature (most notably being Hinata, Kageyama, Nishinoya, and Tanaka) and sometimes other teams because of his casual smug attitude, taking pride in and relishing when his opponents see him as a nuisance as seen with Tendō and Shirabu.
Whenever someone does or says something stupid or silly he is usually seen snickering in the background. However, he does seem to be intimidated by his seniors, being nowhere in sight when Daichi reprimands Tanaka for antagonizing Aoba Josai even though he did so as well, or backing down whenever Ennoshita calls him out.
Despite coming across as arrogant, Tsukishima has low self-esteem and thinks of himself as inferior. This is first noted to be a surprise by Kuroo, who accidentally riles him up by suggesting he’ll lose to Hinata, to which he says that he’s already below him. After the Shiratorizawa match, despite being the MVP, he’s seen calling himself lame for only being able to block Ushijima’s spikes once, only adding to how little he believes in himself and his achievements.
Although Tsukishima maintains a persona of someone who shows little interest in anything and wouldn't do something he didn't like or enjoy, he does truly enjoy volleyball. He initially refrained from putting too much effort into volleyball and being too passionate about it out of fear of suffering from the same fate as his brother. However, after his confrontation with Yamaguchi and hearing Bokuto's inspirational words in the Tokyo training camp, he started to take the sport more seriously and finally found his passion and motivation for volleyball when he blocked Ushijima.
Tsukishima keeps his cool, for the most part, but he can be seen getting visibly annoyed when people underestimate him or tell him what he can or can not do, and uses it as motivation to prove them wrong. He also says things like "It's not a competition" with Hinata but then immediately proceeds to do something to make him jealous, such as the time difference attack during the Shiratorizawa match.
According to Akiteru, "Kei is blunt and doesn't show his emotions often, but he is actually a very hardworking and kind boy."
Tsukishima is shown to be immensely proficient at blocking primarily because of his height. It has been said by Kenma that Tsukishima is a calm and collected blocker who thoroughly analyzes his actions, quite contrary to Hinata's behavior. His ability also allows him to perform split-second feints to surprise and put his opponents off-balance. After the final training camp before the spring tournament, he is able to match the power and speed of an adult, whereas before he couldn't even hold the block.
Height and reach, as of mid-November::
- Fingertip Height: 248 cm
- Jumping Reach: 334 cm (spike) / 325 cm (block)
- Blocking: Tsukishima has established himself as the blocking cornerstone of the team and is often trusted to stop opponents' powerful spikes. He learns most of his blocking skills from Tetsurō Kuroo, who is known to be a veteran in blocking, during the Tokyo summer training camp. There, he learns about putting power into the tips of his fingers and extending his hands out in front of him instead of above him. In addition, he has practiced with his brother's college team to match the timing of his opponents' spike. In matches, he has become the team's blocking playmaker and directs other blockers in his line. With a height of 190cm, Tsukishima has proven to be a troublesome opponent to many spikers. He is said to be the “freak quick killer” by Coach Ukai simply because of how Tsukishima has had to constantly deal with the quick, and as such has gotten used to and sick of it altogether.
- Logical Mind: According to Akiteru, Tsukishima isn't the sort of player who moves on instinct. He doesn't trust in his own strength, nor in the senses, he's honed. The only thing he trusts is the "information" in front of him. As such, he rarely falls for opponents' decoys, including the One Person Time Difference attack. He is quick to process all the players' information and deduce where the toss will go to put up a strong block. This ability makes Tsukishima an exceptional read blocker. In addition, Tsukishima has shown an incredible ability to stay cool-headed. During the Inarizaki match, he persists in restricting Suna's spiking course for the benefits of his team despite many other admitting that they can't resist going for a direct block. Takeda has referred to him as "Karasuno's rational mind." 
- Feint: Tsukishima has used a strategy in which he repeatedly feints to trick his opponents into moving forward. Once that happens, he would change to a normal spike aimed at the newly empty space on the court. 
- One Person Time Difference Attack: It is revealed that Tsukishima acquires this skill practicing with Akaashi during the Tokyo summer training camp. He successfully pulls off this attack during the Shiratorizawa match, in which he tricks the blocker with a delayed jump before spiking.
- Tossing: During the 2-on-2 matches in the Miyagi First Years' Training Camp, Tsukishima is forced to toss. Initially, his toss is too low and sloppy, but he quickly made adjustments after complaints from Goshiki. He can now send high tosses to Asahi, although he still credits it's success mostly to luck.
- Favorite Food: Strawberry shortcake.
- Current Concern: Upon entering high school, he once again has to put up with various people asking how to read the character for his first name.
- The character for his name (蛍) means firefly and can be read as either Hotaru or Kei.
- Tsukishima wears white gym shoes with green accents.
- His star sign is Libra.
- He has the third-smallest appetite in the series, often making his teammates and upperclassmen worry for his health.
- His BMI is the lowest of the male characters in the series.
- He has a habit of fiddling with his fingers when he feels nervous, which is demonstrated most notably when he asks Coach Ukai for advice on blocking.
- He is usually seen with his white Sony (Somy in anime, to avoid copyright issues) headphones around his neck whenever he's not practicing or playing.
- Tsukishima appears to be good at studying, as he belongs to Class 1-4, known to be a college preparatory class.
- His birthday falls on the day of the Harvest Moon–which is when the Mid-Autumn Festival, also known as the Moon Festival, on the Chinese calendar is celebrated in September 2015. Coincidentally, the first character in his name means "moon". In addition, Tsukishima was often seen with a crescent moon symbol on the back of his hoodies and jackets as a child.
- He keeps toy dinosaurs on a shelf above his desk. Whether he actually likes dinosaurs or not is unknown, but it can be assumed that he does, as is demonstrated in a Let's! Haikyuu!? chapter.
- Tsukishima is often referred to as the moon while Hinata is the sun due to their surnames and opposing personalities.
- In a poll conducted by Charapedia, a Japanese anime and manga character database, that asked which anime characters would they choose as the basis for their children's name, Tsukishima came in 9th with 98 votes among female respondents.
- In Haikyū!!: One Shot, Tsukishima was a second-year while Hinata and Kageyama were first years. He was also only 184 cm.
- Furudate picked Tsukishima's name because “His last name has ‘moon’ (月) to contrast Hinata’s ‘sun’ (日). But I named him ‘firefly’ (蛍) because he can emit light on his own.”
- In Haikyuu's first popularity poll, Tsukishima placed 8th with 3,407 votes. In the second, he rose to 4th with 8,305 votes.
- Kei (蛍) - Firefly
- Tsukishima (月島) - Moon Island