No problem! topic.posters.1.user is what you’re looking for (the first reply). If it exists, use that, else default to the OP.
This is how it’s currently structured in Central—
{{#if topic.posters.1.user}}
<span class="tli__last-reply">
{{d-icon "reply"}}
<a href="{{topic.lastPoster.user.userPath}}" data-user-card="{{topic.lastPoster.user.username}}">
{{~topic.lastPoster.user.username~}}
</a>
{{theme-i18n "topic_list_item.replied"}}
<a href={{topic.lastPostUrl}}>
{{format-date topic.bumpedAt format="medium" leaveAgo="true"}}
</a>
</span>
{{else}}
<span class="tli__last-reply">
{{d-icon "m-post_add"}}
<a href="{{topic.posters.0.user.userPath}}" data-user-card="{{topic.posters.0.user.username}}">
{{~topic.posters.0.user.username~}}
</a>
posted
<a href={{topic.lastPostUrl}}>
{{format-date topic.bumpedAt format="medium" leaveAgo="true"}}
</a>
</span>
{{/if}}