You can call someone who can correct a character's stroke mistake your teacher
Often used to refer to someone who excels at correcting or improving written works
Positive
我的作文经叔叔稍稍一改果然完美了好多，真是我的一字之师 | Wǒ de zuòwén jīng shūshu shāoshāo yī gǎi guǒrán wánměi le hǎoduō, zhēnshi wǒ de yīzìzhīshī
Sure enough, my paper improved substantially with a little help from my uncle. He's really good at editing!
[Source]
李明是我的同学，又是我的一字之师，我的好多作文都是经他手改的 | Lǐ Míng shì wǒ de tóngxué, yòu shì wǒ de yīzìzhīshī, wǒ de hǎoduō zuòwén dōu shì jīng tā shǒu gǎi de
Li Ming is my classmate, but also great with writing. Lots of my papers have been improved by him.
[Source]
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一字师 (yī zì shī)
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《五代史补》 (Wǔ Dài Shǐ Bǔ), a collection by 陶岳 (Táo Yuè) of the Northern Sòng Dynasty (北宋 | běi sòng | 960 - 1127). It details events of the tumultuous Fve Dynasties and Ten Kingdoms period ( 五代十国 | wǔ dài shí guó | 902 - 979)
During the Táng Dynasty (唐 | 618 - 907) there lived a Buddhist monk named 齐己 (Qí Jǐ). 齐己 was an excellent poet, and one harsh winter, he was inspired to write about plum blossoms he saw blooming despite the snow. Part of his poem included the line "前村深雪里，昨夜数枝开," which roughly translates to "The branches of the plum trees by the village hang heavy beneath the snow, and yet last night, flowers blossomed.
齐己 had a good friend named 郑谷 (Zhèng Gǔ) who was also an excellent poet. After 齐己 finished his poem about the blossoms, 郑谷 took a look, and after some consideration, he suggested that 齐己 change "数" to "一" in the aforementioned line so that it read "昨夜一枝开." This small change would alter the meaning to something like "The branches of the plum trees by the village hang heavy beneath the snow, and yet last night, a single flower blossomed." 郑谷 figured that this small distinction would emphasize the rarity of the flower blooming in the winter, as it wouldn't be rare for a flower to bloom if other flowers were also blooming.
齐己 humbly accepted the suggestion and called his friend his "一字之师."