How should backend search terms be optimized on Amazon.co.jp, focusing on character usage?

Backend search terms on Amazon.co.jp should be optimized by maximizing the available character count with relevant, high-converting keywords that customers use when searching for the product, but which aren't already included in the product title or description. The goal is to increase product visibility by covering as many relevant search queries as possible. First, identify relevant keywords in Japanese using keyword research tools and analyze competitor listings to find frequently used search terms. Second, use all available characters efficiently, as each character represents an opportunity to capture more search queries. Third, avoid repeating keywords and focus on using synonyms, related terms, and variations of the product name, including both hiragana, katakana, and kanji versions, if applicable. For instance, if a product is a 'wireless headphone', include variations like 'ワイヤレスヘッドホン' (katakana), '無線ヘッドホン' (kanji), and 'コードレスイヤホン'(related term). Fourth, do not include brand names (unless it's your own), or subjective terms like 'best' or 'cheap.' Fifth, backend keywords should be separated by spaces only, not commas or other punctuation, as this wastes valuable character space. For instance, a good use of space would be '軽量イヤホン 高音質ヘッドフォン スポーツ向け 防水' instead of '軽量イヤホン, 高音質ヘッドフォン, スポーツ向け, 防水'.