Konstantin Obenland

This plugin needs Jetpack by WordPress.com to be activated and its Sharing module to be enabled. 
Enter the desired Twitter usernamen in Settings -> Sharing. It will then be appended, every time a visitor shares your content on Twitter
See the <a href="https://dev.twitter.com/docs/tweet-button" title="Tweet Button" rel="nofollow ugc">Tweet Button documentation</a> for details.
<h4>Translations</h4>
I will be more than happy to update the plugin with new locales, as soon as I receive them! 
Currently available in:
<ul>
<li>English</li>
</ul>

Jetpack Twitter Via

Adds 'via @username' to the Tweet Button provided in "Jetpack by WordPress.com"

Barry Carlyon

This plugin adds extra bits an pieces to <a href="https://wordpress.org/extend/plugins/jetpack/" rel="ugc">WordPress Jetpack</a>
Previous called “Jetpack Extras” renamed at request of Jetpack Team to “Custom Tweaks for Jetpack”
Which includes the following additions:
<ul>
<li>
Ability to control button placement, above, below, or both of the post content, with separate options for the archive page and content display page
</li>
<li>
Twitter Button added Related (username/optional description format)
</li>
<li>Adds the ability to make the Twitter button share the WP.me url, if that JetPack module in use, <a href="http://skipsloan.com/?p=175" rel="nofollow ugc">As suggested by SkipTweets on Twitter</a></li>
</ul>
Currently removed is:
<ul>
<li>Ability to turn on/off the <a href="https://dev.twitter.com/docs/tweet-button#optout" rel="nofollow ugc">DNT Twitter</a> button mode – it is difficult to add without editing chunks of the core</li>
</ul>

Custom Tweaks for Jetpack by BarryCarlyon

Extras for WordPress Jetpack. Tested up to Jetpack 3.4.1

Jetpack Twitter Via vs Custom Tweaks for Jetpack by BarryCarlyon