He has light brown hair parted on the left, and dark eyes (usually shown closed). He is always shown wearing glasses and a suit. He has the overall appearance of a butler; Kirin once mistook him for one by calling him "shitsuji-san". His expression is shown to be stern and poised.
A mysterious and serious man with a dry humor. He shows great dedication to his work and is good at everything, from cooking up a dinner spread to the chairwoman's daily orders. However, he can take things a bit too seriously, such as making an impromptu decision to go on a journey to improve his cooking skills when Kotaro lost his appetite.
He has a habit of adding -sama (a polite term) to the names of those he converses with. He had difficulty interacting with the daycare children while Usaida was sick and on leave. But his usual polite and formal manner of being subservient somehow won them over as they saw themselves as the kids being "in charge" over an adult.
Whenever Ryuuichi or Kotarou commend his actions even the slightest, he is humble, stating that "he is not deserving of the praise", but is grateful for the kind words. He constantly strives to perfect his skills.
He celebrates any little event that happens to the Kashima brothers and seems to be able to understand what Kotarou thought without speaking. He is kind and caring, and sees the Kashima brothers as his own family. He also acts as a bridge between the Chairwoman and the Kashima brothers as he explains to the brothers that the Chairwoman is a shy person who is gruff but actually very kind and embarrassed about it. He can act very wise and insightful.
However, most of the time he is an enigmatic character with an unknown background and acts very strange at times (at one point when Usaida went out for drinks with him the store owner thought he was a mannequin). He is friendly, open to others, caring, mysterious and eccentric. He has superhuman abilities and is capable of doing anything.
Nothing is known about his past other than that he has worked for the chairwoman's family for years, even when the Chairwoman's son and daughter-in-law were still alive. He appears to be a studious bookworm when he was a teenager and lived at some point with his grandmother.
Chairwoman: He works as her secretary but also cooks and cleans. He understands her kind nature and how despite her gruff appearance and blunt personality, she is actually caring and embarrassed to show her softer side. The Chairwoman has faith in Saikawa's abilities but is annoyed by his overt attempts at getting her to express herself more and is exasperated by his eccentric behaviour.
Kashima brothers: He gets along well with both and respect them as much as he respects the Chairwoman. He cares for them and when he feels he has let them down he will do whatever it takes to better himself (even when he doesn't need to with only Ryuuichi's insistence that everything is fine being the only thing to stop him)—even when sick, he constantly chooses to "reflect" on how the experience can help him better care for them. Ryuuichi has noted that Saikawa has a weird ability to understand what Kotarou means, even when the toddler is incomprehensible.
Usaida: They both have drinks together once. While Usaida was sick, Usaida trusted Saikawa to watch over the children at the daycare. Nevertheless, Usaida remains clueless about Saikawa's likes and dislikes, or his family in the past.
- the character 犀 / Sai in his name means Rhinoceros
- As a recurring gag, Saikawa makes sekihan (red bean rice for special occasions) after hearing that Ryuuichi has supposedly found his first love and for other important love matters.
- He has a tendency of closing his eyes throughout the series even in photos, and opens a single eye when observing.
- His favourite food is of French origin — pain de campagne (sourdough bread), hachis parmentier (shephard's pie), gratin dauphinois (traditional dish with potatoes and creme fraiche) — made by his grandmother.
- The Babysitter Club once wondered what Saikawa's home was like. Ryuuichi couldn't imagine him living in a regular apartment, which led to the others letting their imaginations run wild.
- Hayato imagined he spent his off-duty hours inside of a coffin.
- Usaida imagined that there was a manufacturing plant that produced many Saikwawas daily, and they never encountered the same one twice.
- Kirin believed he lived in a castle with a princess.
- Taka imagined he lived in a secret base, complete with its very own monster.
- Takuma and Kazuma thought he lived in a spaceship.
- Gakuen Babysitters, chapter 27.1
- Gakuen Babysitters, chapter 72
- Gakuen Babysitters, chapter 40