Unfortunately, Firebase presents aggregated data, in most cases. Secondly, in case of custom events, value will not be available in the dashboard for you to see.
Your best shot if you are going for free is to use pre-existing event template and fit your events to suit your need, such as:
static func levelChange(level: Int, gameIndex: Int) {
Analytics.logEvent(AnalyticsEventLevelUp, parameters: [AnalyticsParameterLevel: level as NSNumber,
AnalyticsParameterCharacter: String(gameIndex) as NSString])
}
Here I have used the pre-existing event template, though my event is not actually a level-up, but at least I am able to see values (aggregated) in the dashboard, as below:

Alternatively, which I have now moved onto, is to activate Blaze plan within Firebase, wherein all your raw events will be pushed to BigQuery DB, from where you can access whatever custom event parameters you've stored.