Shijou Saikyou no Deshi Kenichi

From Shogakukan:  
Kenichi Shirahama, a hopelessly ordinary high school student, was guided by Miu—an incredibly strong young girl—and now spends his days training at Ryouzanpaku, a place where masters of various martial arts and combat styles gather! Just as Kenichi has grown and become stronger by overcoming incredible trials, a new enemy makes an appearance... Could it be a ninja?! And to top it all off, his old companions and his sworn enemy, YOMI, have also returned...

Ryouzanpaku needs money and Shigure takes a job offer where she has to tutor a little foreign girl for one week. She doesn't only teach her martial arts, but also how to make friends.

(Source: Kenichi Wikia)

From Mangadex:  
Spin-off from History's Strongest Disciple Kenichi. Note: Only read this manga once you're up to at least chapter 313 of Kenichi main series. Released at the same time as Kenichi volume 48 (containing chapters 448-457), Kenichi Plus is a collection of three short stories that will take us back in time… The first story features Tanimoto, as he kindly reminisces his memories of Kano Shou, from the first time he met him, up until Kano Shou last fight… In the second story, we follow Loki and No.20 adventures, as they start and struggle to run their detective agency. But that's when Niijima shows up with a request that fit them perfectly. For the final story, which features Renka, we will follow Pisuke's story, a bird who lived many adventures before ending up as Renka's pet

Tatakae! Ryouzanpaku Shijou Saikyou no Deshi, relates the story of a peaceful yet weak boy, who one day, after being beaten up really bad by some of his school comrades, decides that being bullied around isn't an option anymore. Seeking revenge on his bullies, the young boy, Shirahama Kenichi, decides to join a fighting dojo. But what he doesn't know is that the dojo he wants to join is actually a place of gathering for those who have mastered different types of fighting styles. Thus begins his journey...