I'm missing some important parts that I would argue any (binary)format needs.

  - a magic header
  - a version number
  - a crc or data corruption check

Even the json example provided at the bottom can be minified from 418 characters to 166 by replacing the field names with single characters and removing the spaces. Almost all of the savings in this format come from not including the the field names and having a position dependent layout. You can choose to have delimiters or arrange for a byte to indicate the type(+size) for example the following string encodes the example data almost (85bytes vs 80bytes) as efficient but is still readable and supports utf-8 interpretation.

  123456789;LeroyJenkins;60;alliance;p,100,200,300;i,999,1,1;i,45,100,0;a,s,120;a,a,45;

  { "itemId": 999, "quantity": 1, "isSoulbound": false }
  i,999,1,0

  default = { "itemId": 0, "quantity": 1, "isSoulbound": false }
  { "itemId": 999}
  i,999