This character is a Modifier Letter and is mainly used in the Hiragana script.
The glyph is not a composition. Its East Asian Width is wide. In bidirectional text it is written from left to right. When changing direction it is not mirrored. U+309D prohibits line breaks before its position. There might be some exceptions, though.
The Wikipedia has the following information about this codepoint:
Iteration marks are characters or punctuation marks that represent a duplicated character or word.
Representations
System
Representation (click value to copy)
Nº
12445
UTF-8
E3 82 9D
UTF-16
30 9D
UTF-32
00 00 30 9D
URL-Quoted
%E3%82%9D
HTML hex reference
ゝ
Wrong windows-1252 Mojibake
â—Œã‚
Encoding: BIG5 (hex bytes)
C6 A2
Encoding: BIG5HKSCS (hex bytes)
C6 DC
Encoding: CP932 (hex bytes)
81 54
Encoding: CP950 (hex bytes)
C6 A2
Encoding: EUC_JP (hex bytes)
A1 B5
Encoding: EUC_JIS_2004 (hex bytes)
A1 B5
Encoding: EUC_JISX0213 (hex bytes)
A1 B5
Encoding: GBK (hex bytes)
A9 66
Encoding: GB18030 (hex bytes)
A9 66
Encoding: ISO2022_JP (hex bytes)
1B 24 42 21 35 1B 28 42
Encoding: ISO2022_JP_1 (hex bytes)
1B 24 42 21 35 1B 28 42
Encoding: ISO2022_JP_2 (hex bytes)
1B 24 42 21 35 1B 28 42
Encoding: ISO2022_JP_2004 (hex bytes)
1B 24 42 21 35 1B 28 42
Encoding: ISO2022_JP_3 (hex bytes)
1B 24 42 21 35 1B 28 42
Encoding: ISO2022_JP_EXT (hex bytes)
1B 24 42 21 35 1B 28 42
Encoding: SHIFT_JIS (hex bytes)
81 54
Encoding: SHIFT_JIS_2004 (hex bytes)
81 54
Encoding: SHIFT_JISX0213 (hex bytes)
81 54
Adobe Glyph List
iterationhiragana
digraph
*5
RFC 5137
\u'309D'
Bash and Zsh inside echo -e
\u309D
C and C++
\u309D
C#
\u309D
CSS
\00309D
Excel
=UNICHAR(12445)
Go
\u309D
JavaScript
\u309D
Modern JavaScript since ES6
\u{309d}
JSON
\u309D
Java
\u309D
Lua
\u{309D}
Matlab
char(12445)
Perl
"\x{309D}"
PHP
\u{309d}
PostgreSQL
U&'\309D'
PowerShell
`u{309D}
Python
\u309D
Ruby
\u{309d}
Rust
\u{309d}
Click the star button next to each label to set this representation as favorite or remove it from the favorites. Favorites will be shown initially. (Favorites are stored locally on your computer and never sent over the internet.)