For users running Graylog and ES2, the requirement to install Kibana to use Marvel is a bit over the top. This worked perfectly fine in 1.3 releases.
What is the option without having to install Kibana just have the plugin work?
UI code has no business living inside of Elasticsearch. On the other hand, the Marvel 2.x UI plugin was written for Kibana because it's a natural place for UI code to live.
The marvel-agent plugin works without the UI, but to visualize the data automatically, you will need Kibana and the marvel UI plugin. For what it's worth, Kibana is lightweight.