Goshiki Tsutomu

What's interesting is that the romance would be slow not because they weren't compatible, but because they would both be very focused on their athletic goals. However, they would share a mutual understanding that few people their age possess: knowing exactly what it takes to pursue a competitive dream every day. That would likely make Goshiki respect her even before he began to see her romantically.

In this case, she does athletics:
More mutual respect

She would be one of the few people who would understand perfectly:

*Constant muscle soreness.

• Accumulated fatigue.
  Injuries. The pressure of competing.
  The fear of performing poorly.

Yes, it is quite likely that a romance between Tsutomu Goshiki and Kenjirō Shirabu's younger sister would progress very slowly if she had a personality as reserved, shy, and dry as her brother's.

In fact, their personalities could make the beginning especially complicated:

Goshiki usually expresses what she feels, although sometimes in an awkward or exaggerated way. A very shy and inexpressive girl would probably respond with short phrases, few gestures, and few clear signals.
Goshiki might interpret that behavior as disinterest, even if she was simply introverted. For example, imagine this exchange: Goshiki: “Did you enjoy today’s match?” Her: “Yes.” Goshiki: “And what did you think of my smash in the second set?” Her: “It was good.” Goshiki (inside): 'It was good'? Is that good? Is it bad? Does she not care? Meanwhile, she might be thinking: “It was amazing… but I don’t know how to say it.” Also, if she resembles Shirabu, she is likely to be observant.
*Speak little.

• Don't easily express your emotions.
  Prefer to show affection through actions rather than words. Be practical and direct.

Interestingly, that personality might end up attracting Goshiki. He's used to his classmates reacting to his grand pronouncements, so someone who simply looks at him and says "well done" might seem mysterious and interesting to him.

Progress would probably look something like this:

1. Goshiki tries to get her attention.
2. She responds minimally.
3. Goshiki gets confused.
4. They start to coincide more.
5. He discovers that she does pay attention to everything he does.
6. She begins to show small signs of confidence.
7. Goshiki realizes that these small gestures mean a lot.

It would be a romance of small gestures rather than grand declarations. A girl like that probably wouldn't say "I like you" easily. Perhaps she'd show her affection by remembering an important game.

If we imagine Tsutomu Goshiki in a relationship with the sister of one of his teammates, he would probably behave in a rather characteristic way:

He would constantly try to impress her. Goshiki has a strong need to prove himself, so he would definitely talk about his training, his achievements, and how one day he'll be the team's star player. On dates, I imagine him as someone who: plans things enthusiastically.
He tries very hard to appear chivalrous. He remembers important details.
She gets very excited when everything goes well. She can be easily embarrassed by public displays of affection, especially at first.

In summary, Goshiki would probably be a loving, hardworking, and very dedicated boyfriend, although also somewhat insecure and in need of reassurance due to his youth and his constant desire to prove that he lives up to expectations.
He would get nervous easily. Although he tries to appear confident, when something really matters to him he tends to blush, stumble over his words, or act awkwardly. He would be very loyal. Once committed to someone, he's the type of person who would put in a lot of effort to make the relationship work.
He would seek approval. His partner's opinion would matter a great deal to him, and he would likely feel especially proud when she praised his play or acknowledged his efforts. He would be protective, but not controlling. Rather than controlling, he would want to help and be there when she needed him.
There would be some dramatic moments. If they argued, or if he felt he had disappointed his partner, it's easy to imagine him dwelling on the matter and worrying more than necessary.

If the girl were the sister of one of Shiratorizawa's teammates, she would probably be very careful not to cause any problems within the team at first. It's also possible that she would try to act more mature than she actually is to make a good impression on her brother.

Strengths

*Very powerful shots.

• Good jump.
  *High offensive aggression.
• Competitive mentality.
  Constant desire to improve.

What stands out most is his desire to become Shiratorizawa's next ace, following in the footsteps of Wakatoshi Ushijima. He takes this goal very seriously and works tirelessly to achieve it.

Outside of games, he is much less intimidating.

Personality

Enthusiastic. Hardworking.
Proud. Dramatic.
A bit childish for his age. Very respectful to his senpai.

He has a habit of proclaiming himself as the future ace of the team, but the older players usually take it with humor.

He often tries to act like a cool, confident player, even though he's really seeking recognition and validation.

With Ushijima

He deeply admires Wakatoshi Ushijima.

He wants to follow in his footsteps and be a worthy successor to the position of ace.

With Satori Tendō

Tendō often enjoys teasing him a little because of how easy it is to provoke reactions in him.

With the veterans

He respects them a lot and listens to their advice, although sometimes he tries too hard to impress them.

He enjoys training and improving his skills. He dedicates a large part of his free time to volleyball.
He is interested in activities related to sports and physical improvement.

How others perceive it

Most of his colleagues see him as:

*Very talented.

• Somewhat conceited.
  Surprisingly adorable when he loses his temper. A player with enormous potential.

In fact, a significant part of his charm as a character is the contrast between how he tries to present himself—as a confident future superstar—and how he really is: a young, passionate, hardworking, and still insecure boy who is learning to believe in himself.

He can get nervous under pressure. He sometimes loses confidence when things go wrong.
*He tends to exaggerate his abilities.

• His experience is less than that of veteran players.

Although he tries to appear unperturbed, he is quite sensitive emotionally.